View source for Argon compounds

{{short description|Class of chemical compounds}}
'''Argon compounds''', the chemical compounds that contain the element [[argon]], are rarely encountered due to the [[Inert gas|inertness]] of the [[argon]] atom. However, compounds of argon have been detected in [[inert gas matrix]] isolation, cold gases, and plasmas, and [[molecular ion]]s containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar<sub>1</sub>C<sub>60</sub> is stable at room temperature. Ar<sub>1</sub>C<sub>60</sub> was discovered by the [[CSIRO]].

Argon ionises at 15.76&nbsp;eV, which is higher than hydrogen, but lower than helium, neon or fluorine.<ref name=Schilke/> Molecules containing argon can be [[van der Waals molecule]]s held together very weakly by [[London dispersion force]]s. Ionic molecules can be bound by charge induced dipole interactions. With gold atoms there can be some covalent interaction.<ref name=gold>{{cite journal|last1=Shayeghi|first1=Armin|last2=Johnston|first2=Roy L.|last3=Rayner|first3=David M.|last4=Schäfer|first4=Rolf|last5=Fielicke|first5=André|title=The Nature of Bonding between Argon and Mixed Gold–Silver Trimers|journal=Angewandte Chemie International Edition|date=1 September 2015|volume=54|issue=36|pages=10675–10680|doi=10.1002/anie.201503845|pmid=26206667|s2cid=41139746 |url=http://pure-oai.bham.ac.uk/ws/files/27992523/ANIE_accepted.pdf}}</ref> Several boron-argon bonds with significant covalent interactions have been also reported.<ref name=":0">{{Cite journal|last1=Zhou|first1=Mingfei|last2=Wang|first2=Guanjun|last3=Liu|first3=Yuhong|last4=Li|first4=Wei|last5=Jin|first5=Jiaye|date=2017-08-21|title=Preparation and characterization of chemically bonded argon–boroxol ring cation complexes|journal=Chemical Science|language=en|volume=8|issue=9|pages=6594–6600|doi=10.1039/C7SC02472J|pmid=28989687|pmc=5627188|issn=2041-6539}}</ref><ref name=":1">{{Cite journal|last1=Warneke|first1=Jonas|last2=Jenne|first2=Carsten|last3=Wang|first3=Xue-Bin|last4=Asmis|first4=Knut R.|last5=Xantheas|first5=Sotiris S.|last6=Grabowsky|first6=Simon|last7=Azov|first7=Vladimir A.|last8=Aprà|first8=Edoardo|last9=Exner|first9=Rüdiger M.|date=2019-04-23|title=Rational design of an argon-binding superelectrophilic anion|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=17|pages=8167–8172|doi=10.1073/pnas.1820812116|issn=0027-8424|pmid=30952786|pmc=6486711|bibcode=2019PNAS..116.8167M |doi-access=free}}</ref> Experimental methods used to study argon compounds have included [[inert gas matrix|inert gas matrices]], [[infrared spectroscopy]] to study [[Molecular vibration|stretching and bending movements]], [[microwave spectroscopy]] and [[far infrared]] to study rotation, and also visible and [[ultraviolet spectroscopy]] to study different electronic configurations including [[excimer]]s. [[Mass spectroscopy]] is used to study ions.<ref name="Grills"/> Computation methods have been used to theoretically compute molecule parameters, and predict new stable molecules. Computational ''ab initio'' methods used have included [[Coupled cluster|CCSD(T)]], MP2 ([[Møller–Plesset perturbation theory]] of the second order), CIS and [[Configuration interaction singles & doubles|CISD]]. For heavy atoms, [[Effective Core Potential|effective core potentials]] are used to model the inner electrons, so that their contributions do not have to be individually computed. More powerful computers since the 1990s have made this kind of ''[[in silico]]'' study much more popular, being much less risky and simpler than an actual experiment.<ref name="Grills"/> This article is mostly based on experimental or observational results.

The [[argon fluoride laser]] is important in [[photolithography]] of silicon chips. These lasers make a strong ultraviolet emission at 192&nbsp;nm.<ref>{{Cite journal|last1=La Fontaine|first1=B.|url=http://spie.org/x42152.xml|title=Lasers and Moore's Law|journal=SPIE Professional|date=October 2010|page=20}}</ref>

==Argonium==
{{main|Argonium}}
[[Argonium]] (ArH<sup>+</sup>) is an ion combining a proton and an argon atom. It is found in interstellar space in diffuse [[atomic hydrogen]] gas where the fraction of [[molecular hydrogen]] H<sub>2</sub> is in the range of 0.0001 to 0.001.<ref name=Schilke>{{cite journal|last1=Schilke|first1=P.|last2=Neufeld|first2=D. A.|last3=Müller|first3=H. S. P.|last4=Comito|first4=C.|last5=Bergin|first5=E. A.|last6=Lis|first6=D. C.|last7=Gerin|first7=M.|last8=Black|first8=J. H.|last9=Wolfire|first9=M.|last10=Indriolo|first10=N.|last11=Pearson|first11=J. C.|last12=Menten|first12=K. M.|last13=Winkel|first13=B.|last14=Sánchez-Monge|first14=Á.|last15=Möller|first15=T.|last16=Godard|first16=B.|last17=Falgarone|first17=E.|title=Ubiquitous argonium (ArH<sup>+</sup>) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas|journal=Astronomy & Astrophysics|date=4 June 2014|volume=566|pages=A29|doi=10.1051/0004-6361/201423727|bibcode=2014A&A...566A..29S|arxiv=1403.7902|s2cid=44021593}}</ref>

Argonium is formed when [[Dihydrogen cation|H<sub>2</sub><sup>+</sup>]] reacts with Ar atoms:<ref name=Schilke/>

:Ar + {{chem|H|2|+}} → ArH<sup>+</sup> + H<ref name=Schilke/>

and it is also produced from Ar<sup>+</sup> ions produced by cosmic rays and X-rays from neutral argon:

:Ar<sup>+</sup> + H<sub>2</sub> → *ArH<sup>+</sup> + H<ref name=Schilke/> 1.49&nbsp;eV.<ref name=barlow>{{cite journal|last1=Barlow|first1=M. J.|last2=Swinyard|first2=B. M.|last3=Owen|first3=P. J.|last4=Cernicharo|first4=J.|last5=Gomez|first5=H. L.|last6=Ivison|first6=R. J.|last7=Krause|first7=O.|last8=Lim|first8=T. L.|last9=Matsuura|first9=M.|last10=Miller|first10=S.|last11=Olofsson|first11=G.|last12=Polehampton|first12=E. T.|title=Detection of a Noble Gas Molecular Ion, <sup>36</sup>ArH<sup>+</sup>, in the Crab Nebula|journal=Science|date=12 December 2013|volume=342|issue=6164|pages=1343–1345|doi=10.1126/science.1243582|pmid=24337290|bibcode=2013Sci...342.1343B|arxiv=1312.4843|citeseerx=10.1.1.749.8173|s2cid=37578581}}</ref>

When ArH<sup>+</sup> encounters an electron, dissociative recombination can occur, but it is extremely slow for lower energy electrons, allowing ArH<sup>+</sup> to survive for a much longer time than many other similar protonated cations.

:ArH<sup>+</sup> + e<sup>−</sup> → ArH* → Ar + H<ref name=Schilke/>

Artificial ArH<sup>+</sup> made from earthly Ar contains mostly the isotope <sup>40</sup>Ar rather than the cosmically abundant <sup>36</sup>Ar. Artificially it is made by an electric discharge through an argon-hydrogen mixture.<ref name=brown>{{cite journal|last1=Brown|first1=John M.|last2=Jennings|first2=D. A.|last3=Vanek|first3=M.|last4=Zink|first4=L. R.|last5=Evenson|first5=K. M.|title=The pure rotational spectrum of ArH<sup>+</sup>|journal=Journal of Molecular Spectroscopy|date=April 1988|volume=128|issue=2|pages=587–589|doi=10.1016/0022-2852(88)90173-7|bibcode=1988JMoSp.128..587B|url=https://zenodo.org/record/1253940}}</ref>

===Natural occurrence===
In the [[Crab Nebula]], ArH<sup>+</sup> occurs in several spots revealed by [[emission line]]s. The strongest place is in the Southern Filament. This is also the place with the strongest concentration of Ar<sup>+</sup> and Ar<sup>2+</sup> ions.<ref name=barlow/> The [[column density]] of ArH<sup>+</sup> in the Crab Nebula is between 10<sup>12</sup> and 10<sup>13</sup> atoms per square centimeter.<ref name=barlow/> Possibly the energy required to excite the ions so that then can emit, comes from collisions with electrons or hydrogen molecules.<ref name=barlow/> Towards the Milky Way centre the column density of ArH<sup>+</sup> is around {{val|2|e=13|u=cm<sup>−2</sup>}}.<ref name=Schilke/>

==Cluster argon cations==
The [[diargon]] cation, {{chem|Ar|2|+}} has a binding energy of 1.29&nbsp;eV.<ref name=Linnartz/>

The triargon cation {{chem|Ar|3|+}} is linear, but has one Ar−Ar bond shorter than the other. Bond lengths are 2.47 and 2.73 [[ångström]]s. The dissociation energy to Ar and Ar<sub>2</sub><sup>+</sup> is 0.2&nbsp;eV. In line with the molecule's asymmetry, the charge is calculated as +0.10, +0.58 and +0.32 on each argon atom, so that it greatly resembles {{chem|Ar|2|+}} bound to a neutral Ar atom.<ref name="bowers">{{cite journal|last1=Bowers|first1=Michael T.|last2=Palke|first2=William E.|last3=Robins|first3=Kathleen|last4=Roehl|first4=Coleen|last5=Walsh|first5=Sherrie|title=On the structure and photodissociation dynamics of {{chem|Ar|3|+}}|journal=Chemical Physics Letters|date=May 1991|volume=180|issue=3|pages=235–240|doi=10.1016/0009-2614(91)87146-3|bibcode=1991CPL...180..235B}}</ref>

Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear. {{chem|Ar|13|+}} [[icosahedral]] clusters have an {{chem|Ar|3|+}} core, whereas {{chem|Ar|19|+}} is [[dioctahedral]] with an {{chem|Ar|4|+}} core. The linear {{chem|Ar|4|+}} core has +0.1 charge on the outer atoms, and +0.4 charge on each or the inner atoms. For larger charged argon clusters, the charge is not distributed on more than four atoms. Instead the neutral outer atoms are attracted by induced electric polarization.<ref name=john02/> The charged argon clusters absorb radiation, from the near infrared, through visible to ultraviolet. The charge core, {{chem|Ar|2|+}}, {{chem|Ar|3|+}} or {{chem|Ar|4|+}} is called a [[chromophore]]. Its spectrum is modified by the first shell of neutral atoms attached. Larger clusters have the same spectrum as the smaller ones. When photons are absorbed in the [[chromophore]], it is initially [[excited state|electronically excited]], but then energy is transferred to the whole cluster in the form of [[vibrational energy|vibration]]. Excess energy is removed by outer atoms [[evaporating]] from the cluster one at a time. The process of destroying a cluster by light is called [[Photodissociation|photofragmentation]].<ref name=john02/>

Negatively-charged argon clusters are thermodynamically unstable, and therefore cannot exist. Argon has a negative [[electron affinity]].<ref name=john02/>

==Argon monohydride==
Neutral argon hydride, also known as argon monohydride (ArH), was the first discovered noble gas hydride. J.&nbsp;W.&nbsp;C. Johns discovered an emission line of ArH at 767&nbsp;nm and announced the find in 1970. The molecule was synthesized using [[X-ray]] irradiation of mixtures of argon with hydrogen-rich molecules such as H<sub>2</sub>, [[water|H<sub>2</sub>O]], [[methane|CH<sub>4</sub>]] and [[methanol|CH<sub>3</sub>OH]].<ref name="berm">{{cite journal|last1=Berman|first1=Michael|last2=Kaldor|first2=Uzi|last3=Shmulovich|first3=J.|last4=Yatsiv|first4=S.|title=Rydberg states and the observed spectrum of ArH|journal=Chemical Physics|date=December 1981|volume=63|issue=1–2|pages=165–173|doi=10.1016/0301-0104(81)80318-7|bibcode=1981CP.....63..165B}}</ref> The X-ray excited argon atoms are in the 4p state.<ref>{{cite journal|volume=75|number=2|journal=Chemical Physics Letters|date=15 October 1980|title=Excitation Kinetics of ArH* Luminescence in X-Ray Irradiated Argon-Hydrogen Mixtures|first1=J.|last1=Shmulovich|first2=S.|last2=Yatsiv|pages=319–323|doi=10.1016/0009-2614(80)80523-9|bibcode=1980CPL....75..319S}}</ref>

Argon monohydride is unstable in its ground state, 4s, as a neutral inert gas atom and a hydrogen atom repel each other at normal intermolecular distances. When a higher-energy-level ArH* emits a photon and reaches the ground state, the atoms are too close to each other, and they repel and break up. However a [[Van der Waals force|van der Waals]] molecule can exist with a long bond.<ref name="dabIII"/> However, excited ArH* can form stable [[Rydberg molecules]], also known as [[excimer]]s. These Rydberg molecules can be considered as a [[argonium|protonated argon]] core, surrounded by an electron in one of many possible higher energy states.<ref>{{cite journal|last1=Jungen|first1=C.|last2=Roche|first2=A. L.|last3=Arif|first3=M.|title=The Rydberg spectrum of ArH and KrH: calculation by ''R''-matrix and generalized quantum defect theory|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|date=15 August 1997|volume=355|issue=1729|pages=1481–1506|doi=10.1098/rsta.1997.0072|bibcode=1997RSPTA.355.1481J|s2cid=101818571}}</ref>

:Formation: Ar + [[photon|''ν'']] → Ar*;{{pad|2em}} Ar* + H<sub>2</sub> → ArH* + H<ref name="berm"/>

Instead of dihydrogen, other hydrogen containing molecules can also have a hydrogen atom abstracted by excited argon, but note that some molecules bind hydrogen too strongly for the reaction to proceed. For example, [[acetylene]] will not form ArH this way.<ref name="berm"/>

In the van der Waals molecule of ArH, the bond length is calculated to be about 3.6&nbsp;Å and the [[dissociation energy]] calculated to be 0.404&nbsp;kJ/mol (33.8&nbsp;cm<sup>−1</sup>).<ref>{{cite journal|last1=Partridge|first1=Harry|last2=Bauschlicher|first2=Charles W.|title=The dissociation energies of He<sub>2</sub>, HeH, and ArH: A bond function study|journal=Molecular Physics|date=February 1999|volume=96|issue=4|pages=705–710|doi=10.1080/00268979909483006|bibcode=1999MolPh..96..705P}}</ref> The bond length in ArH* is calculated as 1.302&nbsp;Å.<ref name=parker>{{cite journal|last1=Parker|first1=Eric P.|last2=Ortiz|first2=J.V.|title=Electron propagator calculations on the discrete spectra of ArH and NeH|journal=Chemical Physics Letters|date=November 1989|volume=163|issue=4–5|pages=366–370|doi=10.1016/0009-2614(89)85151-6|bibcode=1989CPL...163..366P}}</ref>

The spectrum of argon monohydride, both ArH* and Ar[[deuterium|D]]*, has been studied. The lowest bound state is termed A<sup>2</sup>Σ<sup>+</sup> or 5s. Another low lying state is known as 4p, made up of C<sup>2</sup>Σ<sup>+</sup> and B<sup>2</sup>π states. Each transition to or from higher level states corresponds to a band. Known bands are 3p&nbsp;→&nbsp;5s, 4p&nbsp;→&nbsp;5s, 5p&nbsp;→&nbsp;5s (band origin {{val|17486.527|u=cm<sup>−1</sup>}}<ref name="dabI">{{cite journal|last1=Dąbrowski|first1=I.|last2=Tokaryk|first2=D. W.|last3=Vervloet|first3=M.|last4=Watson|first4=J. K. G.|title=New Rydberg–Rydberg transitions of the ArH and ArD molecules. I. Emission from np states of ArD|journal=The Journal of Chemical Physics|date=1996|volume=104|issue=21|pages=8245|doi=10.1063/1.471578|bibcode=1996JChPh.104.8245D}}</ref>), 6p&nbsp;→&nbsp;5s (band origin {{val|21676.90|u=cm<sup>−1</sup>}}<ref name="dabI"/>) 3dσ&nbsp;→&nbsp;4p, 3dπ&nbsp;→&nbsp;4p (6900&nbsp;cm<sup>−1</sup>), 3dδ&nbsp;→&nbsp;4p (8200–8800&nbsp;cm<sup>−1</sup>), 4dσ&nbsp;→&nbsp;4p ({{val|15075|u=cm<sup>−1</sup>}}), 6s&nbsp;→&nbsp;4p (7400–7950&nbsp;cm<sup>−1</sup>), 7s&nbsp;→&nbsp;4p (predicted at {{val|13970|u=cm<sup>−1</sup>}}, but obscured), 8s&nbsp;→&nbsp;4p ({{val|16750|u=cm<sup>−1</sup>}}), 5dπ&nbsp;→&nbsp;4p ({{val|16460|u=cm<sup>−1</sup>}}), 5p&nbsp;→&nbsp;6s (band origin 3681.171&nbsp;cm<sup>−1</sup>),<ref name=dabII/> 4f&nbsp;→&nbsp;5s ({{val|20682.17}} and {{val|20640.90|u=cm<sup>−1</sup>}} band origin for ArD and ArH), 4f&nbsp;→&nbsp;3dπ (7548.76 and 7626.58&nbsp;ccm<sup>−1</sup>), 4f&nbsp;→&nbsp;3dδ (6038.47 and 6026.57&nbsp;cm<sup>−1</sup>), 4f&nbsp;→&nbsp;3dσ (4351.44&nbsp;cm<sup>−1</sup> for ArD).<ref name="dabIII">{{cite journal|last1=Dąbrowski|first1=I.|last2=Tokaryk|first2=D. W.|last3=Lipson|first3=R. H.|last4=Watson|first4=J. K. G.|title=New Rydberg–Rydberg Transitions of the ArH and ArD Molecules. III. emission from the 4f complexes|journal=Journal of Molecular Spectroscopy|date=May 1998|volume=189|issue=1|pages=110–123|doi=10.1006/jmsp.1997.7524|pmid=9571129|bibcode=1998JMoSp.189..110D}}</ref> The transitions going to 5s, 3dπ&nbsp;→&nbsp;5s and 5dπ&nbsp;→&nbsp;5s, are strongly [[predissociation|predissociated]], blurring out the lines.<ref name=dabII>{{cite journal|last1=Dąbrowski|first1=I.|last2=Tokaryk|first2=D. W.|last3=Watson|first3=J. K. G.|title=New Rydberg–Rydberg Transitions of the ArH and ArD Molecules. II. Emission from ''n''d and ''n''s States to the 4p State|journal=Journal of Molecular Spectroscopy|date=26 November 1997|volume=189|issue=1|pages=95–109|doi=10.1006/jmsp.1997.7523|pmid=9571128|bibcode=1998JMoSp.189...95D}}</ref> In the UV spectrum a continuous band exists from 200 to 400&nbsp;nm. This band is due to two different higher states: B<sup>2</sup>Π&nbsp;→&nbsp;A<sup>2</sup>Σ<sup>+</sup> radiates over 210–450&nbsp;nm, and E<sup>2</sup>Π&nbsp;→&nbsp;A<sup>2</sup>Σ<sup>+</sup> is between 180 and 320&nbsp;nm.<ref name=wund>{{cite journal|last1=Wunderlich|first1=C.|last2=Betz|first2=V.|last3=Bruckmeier|first3=R.|last4=Figger|first4=H.|title=Lifetime measurements on ArH and ArD|journal=The Journal of Chemical Physics|date=1993|volume=98|issue=12|pages=9362|doi=10.1063/1.464416|bibcode=1993JChPh..98.9362W}}</ref> A band in the near infrared from 760 to 780&nbsp;nm.<ref>{{cite journal|last1=Van Hemert|first1=Marc C.|last2=Dohmann|first2=Helmut|last3=Peyerimhoff|first3=Sigrid D.|title=Theoretical study of radiative and predissociative processes in ArH and ArD|journal=Chemical Physics|date=December 1986|volume=110|issue=1|pages=55–66|doi=10.1016/0301-0104(86)85144-8|bibcode=1986CP....110...55V}}</ref>

Other ways to make ArH include a [[Penning ionization|Penning]]-type discharge tube, or other electric discharges. Yet another way is to create a beam of ArH<sup>+</sup> (argonium) ions and then neutralize them in laser-energized [[caesium]] vapour. By using a beam, the lifetimes of the different energy states can be observed, by measuring the profile of electromagnetic energy emitted at different wavelengths.<ref>{{cite journal|last1=Wunderlich|first1=C.|last2=Witzel|first2=B.|last3=Bruckmeier|first3=R.|last4=Figger|first4=H.|title=UV spectra of ArH and ArD as emitted by a neutralized ion beam|journal=Canadian Journal of Physics|date=November 1994|volume=72|issue=11–12|pages=1236–1240|doi=10.1139/p94-159|bibcode=1994CaJPh..72.1236W}}</ref> The E<sup>2</sup>π state of ArH has a radiative lifetime of 40&nbsp;ns. For ArD the lifetime is 61&nbsp;ns. The B<sup>2</sup>Π state has a lifetime of 16.6&nbsp;ns in ArH and 17&nbsp;ns in ArD.<ref name=wund/>

==Argon polyhydrides==
The argon dihydrogen cation {{chem|ArH|2|+}} has been predicted to exist and to be detectable in the [[interstellar medium]]. However it has not been detected {{as of|2021|lc=y}}.<ref name=Theis/> {{chem|ArH|2|+}} is predicted to be [[linear molecular geometry|linear]] in the form Ar−H−H. The H−H distance is 0.94&nbsp;Å. The dissociation barrier is only 2&nbsp;kcal/mol (8&nbsp;kJ/mol), and {{chem|ArH|2|+}} readily loses a hydrogen atom to yield ArH<sup>+</sup>.<ref>{{cite journal|doi=10.1039/F29726800259|title=Ion-Molecule Reactions of the Rare Gases with Hydrogen. Part 1. Diatomics in Molecules. Potential Energy Surface for ArH2+|journal=J. Chem. Soc., Faraday Trans. 2|volume=68|pages=259–280|first1=P. J.|last1=Kuntz|first2=A. C.|last2=Roach|year=1972}}</ref> The force constant of the ArH bond in this is 1.895&nbsp;m[[dyne]]/Å<sup>2</sup> ({{val|1.895|e=12|ul=Pa}}).<ref name=forten>{{cite journal|last1=Fortenberry|first1=Ryan C.|title=Quantum astrochemical spectroscopy|journal=International Journal of Quantum Chemistry|volume=117|issue=2|pages=81–91|date=June 2016|doi=10.1002/qua.25180|doi-access=free}}</ref>

The argon trihydrogen cation {{chem|ArH|3|+}} has been observed in the laboratory.<ref name=Theis>{{cite journal|last1=Theis|first1=Riley A.|last2=Fortenberry|first2=Ryan C.|title=Potential interstellar noble gas molecules: ArOH<sup>+</sup> and NeOH<sup>+</sup> rovibrational analysis from quantum chemical quartic force fields|journal=Molecular Astrophysics|date=March 2016|volume=2|pages=18–24|doi=10.1016/j.molap.2015.12.001|bibcode=2016MolAs...2...18T}}</ref><ref name=baill>{{cite journal|last1=Bailleux|first1=S.|last2=Bogey|first2=M.|last3=Bolvin|first3=H.|last4=Civiš|first4=S.|last5=Cordonnier|first5=M.|last6=Krupnov|first6=A.F.|last7=Tretyakov|first7=M. Y.|last8=Walters|first8=A.|last9=Coudert|first9=L. H.|title=Sub-Millimeter-Wave Spectroscopy of the {{chem|Ar·H|3|+}} and {{chem|Ar·D|3|+}} Ionic Complexes|journal=Journal of Molecular Spectroscopy|date=July 1998|volume=190|issue=1|pages=130–139|doi=10.1006/jmsp.1998.7564|pmid=9645933|bibcode=1998JMoSp.190..130B}}</ref> ArH<sub>2</sub>D<sup>+</sup>, {{chem|ArHD|2|+}} and {{chem|ArD|3|+}} have also been observed.<ref>{{cite journal|last1=Burenin|first1=A. V.|title=Description of the torsional motion in the isotopically asymmetric ionic complexes ArH<sub>2</sub>D<sup>+</sup> and ArD<sub>2</sub>H<sup>+</sup>|journal=Optics and Spectroscopy|date=6 May 2010|volume=108|issue=4|pages=506–511|doi=10.1134/S0030400X1004003X|bibcode=2010OptSp.108..506B|s2cid=121592414}}</ref> The argon trihydrogen cation is planar in shape, with an argon atom off the vertex of a triangle of hydrogen atoms.<ref>{{cite journal|last1=Burenin|first1=A. V.|title=Description of torsional motion in ionic complexes {{chem|ArH|3|+}} and {{chem|ArD|3|+}}|journal=Optics and Spectroscopy|date=4 September 2009|volume=107|issue=2|pages=228–234|doi=10.1134/S0030400X09080116|bibcode=2009OptSp.107..228B|s2cid=122050884}}</ref>

==Argoxonium==
The argoxonium ion ArOH<sup>+</sup> is predicted to be [[bent molecular geometry]] in the 1<sup>1</sup>A′ state. <sup>3</sup>Σ<sup>−</sup> is a triplet state 0.12&nbsp;eV higher in energy, and <sup>3</sup>A″ is a [[triplet state]] 0.18&nbsp;eV higher. The Ar−O bond is predicted to be 1.684&nbsp;Å long<ref name=Theis/> and to have a force constant of 2.988 mdyne/Å<sup>2</sup> ({{val|2.988|e=12|ul=Pa}}).<ref name=forten/>

==ArNH<sup>+</sup>==
ArNH<sup>+</sup> is a possible ionic molecule to detect in the lab, and in space, as the atoms that compose it are common. ArNH<sup>+</sup> is predicted to be more weakly bound than ArOH<sup>+</sup>, with a force constant in the Ar−N bond of 1.866 mdyne/Å<sup>2</sup> ({{val|1.866|e=12|ul=Pa}}). The [[bond angle|angle]] at the nitrogen atom is predicted to be 97.116°. The Ar−N lengths should be 1.836&nbsp;Å and the N−H bond length would be 1.046&nbsp;Å<ref name=forten/><ref>{{cite journal|last1=Novak|first1=Carlie M.|last2=Fortenberry|first2=Ryan C.|title=Theoretical rovibrational analysis of the covalent noble gas compound ArNH<sup>+</sup>|journal=Journal of Molecular Spectroscopy|date=April 2016|volume=322|pages=29–32|doi=10.1016/j.jms.2016.03.003|bibcode=2016JMoSp.322...29N}}</ref>

==Argon dinitrogen cation==
The argon dinitrogen linear cationic complex has also been detected in the lab:
:Ar + {{chem|N|2|+}} → {{chem|ArN|2|+}} {{overset|photodissociation|→}} Ar<sup>+</sup> + N<sub>2</sub>.<ref name=Theis/>
The dissociation yields Ar<sup>+</sup>, as this is a higher-energy state.<ref name=Linnartz>{{cite journal|last1=Linnartz|first1=H.|s2cid=45850131|title=Rotationally Resolved Infrared Spectrum of the Charge Transfer Complex [Ar–N<sub>2</sub>]<sup>+</sup>|journal=Science|date=16 August 2002|volume=297|issue=5584|pages=1166–1167|doi=10.1126/science.1074201|pmid=12183626|bibcode=2002Sci...297.1166L}}</ref> The binding energy is 1.19&nbsp;eV.<ref name=Linnartz/> The molecule is linear. The distance between two nitrogen atoms is 1.1&nbsp;Å. This distance is similar to that of neutral N<sub>2</sub> rather than that of {{chem|N|2|+}} ion. The distance between one nitrogen and the argon atom is 2.2&nbsp;Å.<ref name=Linnartz/> The vibrational band origin for the nitrogen bond in {{chem|ArN|2|+}} (''V''&nbsp;=&nbsp;0&nbsp;→&nbsp;1) is at 2272.2564&nbsp;cm<sup>−1</sup> compared with N<sub>2</sub><sup>+</sup> at 2175 and N<sub>2</sub> at 2330&nbsp;cm<sup>−1</sup>.<ref name=Linnartz/>

In the process of [[photodissociation]], it is three times more likely to yield Ar<sup>+</sup> + N<sub>2</sub> compared to Ar + {{chem|N|2|+}}.<ref name=linn/>

=={{chem|ArHN|2|+}}==
{{chem|ArHN|2|+}} has been produced in a supersonic jet expansion of gas and detected by [[Fourier transform]] [[microwave spectroscopy]].<ref name=baill/> The molecule is linear, with the atoms in the order Ar−H−N−N. The Ar−H distance is 1.864&nbsp;Å. There is a stronger bond between hydrogen and argon than in ArHCO<sup>+</sup>.<ref name="seki">{{cite journal|last1=Seki|first1=Kouji|last2=Sumiyoshi|first2=Yoshihiro|last3=Endo|first3=Yasuki|title=Pure rotational spectra of the {{chem|Ar–HN|2|+}} and the {{chem|Kr–HN|2|+}} ionic complexes|journal=The Journal of Chemical Physics|date=2002|volume=117|issue=21|pages=9750|doi=10.1063/1.1518025|bibcode=2002JChPh.117.9750S}}</ref>

The molecule is made by the following reaction:

:ArH<sup>+</sup> + N<sub>2</sub> → {{chem|ArHN|2|+}}.<ref name="seki"/>

==Bis(dinitrogen) argon cation==
The argon ion can bond two molecules of dinitrogen (N<sub>2</sub>) to yield an ionic complex with a linear shape and structure N=N−{{overset|+|Ar}}−N=N. The N=N bond length is 1.1014&nbsp;Å, and the nitrogen to argon bond length is 2.3602&nbsp;Å. 1.7&nbsp;eV of energy is required to break this apart to N<sub>2</sub> and {{chem|ArN|2|+}}. The band origin of an infrared band due to antisymmetric vibration of the N=N bonds is at 2288.7272&nbsp;cm<sup>−1</sup>. Compared to N<sub>2</sub> it is redshifted 41.99&nbsp;cm<sup>−1</sup>. The ground state rotational constant of the molecule is {{val|0.034296|u=cm<sup>−1</sup>}}.<ref name=linn>{{cite journal|last1=Linnartz|first1=H.|last2=Verdes|first2=D.|last3=Knowles|first3=P. J.|last4=Lakin|first4=N. M.|last5=Rosmus|first5=P.|last6=Maier|first6=J. P.|title=Linear and centrosymmetric N[sub 2]⋯Ar[sup +]⋯N[sub 2]|journal=The Journal of Chemical Physics|date=2000|volume=113|issue=3|pages=895|doi=10.1063/1.481868|bibcode=2000JChPh.113..895L}}</ref>

{{chem|Ar(N|2|)|2|+}} is produced by a supersonic expansion of a 10:1 mixture of argon with nitrogen through a nozzle, which is impacted by an [[electron beam]].<ref name=linn/>

==ArN<sub>2</sub>O<sup>+</sup>==
ArN<sub>2</sub>O<sup>+</sup> absorbs photons in four violet–ultraviolet wavelength bands leading to breakup of the molecule. The bands are 445–420, 415–390, 390–370, and 342&nbsp;nm.<ref name="nizko">{{cite journal|last1=Nizkorodov|first1=S. A.|last2=Maier|first2=J. P.|last3=Bieske|first3=E. J.|title=The infrared spectrum of the N<sub>2</sub>H<sup>+</sup>–He ion-neutral complex|journal=The Journal of Chemical Physics|date=1995|volume=102|issue=13|pages=5570|doi=10.1063/1.469286|url=http://www.chem.uci.edu/~nizkorod/publications/Basel/1995_Bieske_JCP_He-HN2_letter.PDF|bibcode=1995JChPh.102.5570N}}</ref><ref name=n2o>{{cite journal|last1=Bieske|first1=E. J.|last2=Soliva|first2=A. M.|last3=Friedmann|first3=A.|last4=Maier|first4=J. P.|title=Photoinitiated charge transfer in N<sub>2</sub>O<sup>+</sup>–Ar|journal=The Journal of Chemical Physics|date=1992|volume=96|issue=10|pages=7535|doi=10.1063/1.462405|bibcode=1992JChPh..96.7535B}}</ref>

==ArHCO<sup>+</sup>==
ArHCO<sup>+</sup> has been produced in a supersonic-jet expansion of gas and detected by Fabry–Perot-type Fourier transform microwave spectroscopy.<ref name=baill/><ref>{{cite journal|last1=Ohshima|first1=Yasuhiro|last2=Sumiyoshi|first2=Yoshihiro|last3=Endo|first3=Yasuki|title=Rotational spectrum of the Ar–HCO<sup>+</sup> ionic complex|journal=The Journal of Chemical Physics|date=1997|volume=106|issue=7|pages=2977|doi=10.1063/1.473416|bibcode=1997JChPh.106.2977O|doi-access=free}}</ref>

The molecule is made by this reaction

ArH<sup>+</sup> + CO → ArHCO<sup>+</sup>.<ref name="seki"/>

==Carbon dioxide–argon ion==
{{chem|ArCO|2|+}} can be excited to form {{chem|ArCO|2|+}}* where the positive charge is moved from the carbon dioxide part to the argon. This molecule may occur in the upper atmosphere. Experimentally the molecule is made from a low-pressure argon gas with 0.1% [[carbon dioxide]], irradiated by a 150&nbsp;V [[electron beam]]. Argon is ionized, and can transfer the charge to a carbon dioxide molecule.<ref name=illi>{{cite journal|last1=Illies|first1=Andreas J.|last2=Jarrold|first2=M. F.|last3=Wagner-Redeker|first3=W.|last4=Bowers|first4=Michael T.|title=Photoinduced intramolecular charge transfer: photodissociation of carbon dioxide ion (1+)-argon (CO2<sup>+</sup>·Ar) cluster ions|journal=Journal of the American Chemical Society|date=May 1985|volume=107|issue=10|pages=2842–2849|doi=10.1021/ja00296a003}}</ref> The dissociation energy of {{chem|ArCO|2|+}} is 0.26&nbsp;eV.<ref name=illi/>

{{chem|ArCO|2|+}} + CO<sub>2</sub> → Ar + {{chem|CO|2|·CO|2|+}} (yields 0.435&nbsp;eV.)<ref name=illi/>

==van der Waals molecules==
Neutral argon atoms bind very weakly to other neutral atoms or molecules to form [[van der Waals molecule]]s. These can be made by expanding argon under high pressure mixed with the atoms of another element. The expansion happens through a tiny hole into a vacuum, and results in cooling to temperatures a few degrees above absolute zero.  At higher temperatures the atoms will be too energetic to stay together by way of the weak [[London dispersion forces]]. The atoms that are to combine with argon can be produced by evaporation with a laser or alternatively by an electric discharge. The known molecules include AgAr, Ag<sub>2</sub>Ar, NaAr, KAr, MgAr, CaAr, SrAr, ZnAr, CdAr, HgAr, SiAr,<ref>{{cite journal|last1=Jouvet|first1=C.|last2=Lardeux-Dedonder|first2=C.|last3=Martrenchard|first3=S.|last4=Solgadi|first4=D.|title=Fluorescence excitation spectrum of silver–argon van der Waals complex|journal=The Journal of Chemical Physics|date=1991|volume=94|issue=3|pages=1759|doi=10.1063/1.459949|bibcode=1991JChPh..94.1759J}}</ref> InAr, CAr,<ref>{{cite journal|last1=Lei|first1=Jie|last2=Dagdigian|first2=Paul J.|title=Laser fluorescence excitation spectroscopy of the CAr van der Waals complex|journal=The Journal of Chemical Physics|date=2000|volume=113|issue=2|pages=602|doi=10.1063/1.481835|bibcode=2000JChPh.113..602L}}</ref> GeAr,<ref name="ReferenceB">{{cite journal|last1=Tao|first1=Chong|last2=Dagdigian|first2=Paul J.|title=Laser fluorescence excitation spectroscopy of the GeAr van der Waals complex|journal=The Journal of Chemical Physics|date=2003|volume=118|issue=3|pages=1242|doi=10.1063/1.1529662|bibcode=2003JChPh.118.1242T}}</ref> SnAr,<ref name="ReferenceC">{{cite journal|last1=Tao|first1=Chong|last2=Dagdigian|first2=Paul J.|title=Spectroscopic investigation of nonbonding interactions of group-14 atoms with rare gases: The SnAr van der Waals complex|journal=The Journal of Chemical Physics|date=2004|volume=120|issue=16|pages=7512–9|doi=10.1063/1.1665957|pmid=15267664|bibcode=2004JChPh.120.7512T}}</ref> and BAr.<ref>{{cite journal|last1=Yang|first1=Xin|last2=Dagdigian|first2=Paul J.|title=Fluorescence excitation and depletion spectroscopy of the BAr complex: Electronic states correlating with the excited valence B(2s2p<sup>2</sup> <sup>2</sup>D) asymptote|journal=The Journal of Chemical Physics|date=1997|volume=106|issue=16|pages=6596|doi=10.1063/1.473649|bibcode=1997JChPh.106.6596Y}}</ref> SiAr was made from silicon atoms derived from Si(CH<sub>3</sub>)<sub>4</sub>.<ref>{{cite journal|last1=Dedonder-Lardeux|first1=C.|last2=Jouvet|first2=C.|last3=Richard-Viard|first3=M.|last4=Solgadi|first4=D.|title=Fluorescence excitation spectrum of the Si–Ar van der Waals complex|journal=The Journal of Chemical Physics|date=1990|volume=92|issue=5|pages=2828|doi=10.1063/1.457929|bibcode=1990JChPh..92.2828D}}</ref>

In addition to the very weakly bound van der Waals molecules, electronically excited molecules with the same formula exist. As a formula these can be written ArX*, with the "*" indicating an [[excited state]]. The atoms are much more strongly bound with a covalent bond. They can be modeled as an ArX<sup>+</sup> surrounded by a higher energy shell with one electron. This outer electron can change energy by exchanging photons and so can fluoresce. The widely used [[argon fluoride laser]] makes use of the ArF* excimer to produce strong ultraviolet radiation at 192&nbsp;nm. The argon chloride laser using ArCl* produces even shorter ultraviolet at 175&nbsp;nm, but is too feeble for application.<ref>{{cite book|editor-last=Setser|editor-first=D. W.|title=Reactive Intermediates in the Gas Phase: Generation and Monitoring|date=1979|publisher=Academic Press|location=New York|isbn=978-0-12-637450-6|url=https://books.google.com/books?id=DxoSBQAAQBAJ&pg=PA326}}</ref> The argon chloride in this laser comes from argon and chlorine molecules.<ref>{{cite journal|last1=Shuaibov|first1=A. K.|last2=Shimon|first2=L. L.|last3=Shevera|first3=I. V.|last4=Volovich|first4=P. N.|title=A high-frequency UV-VUV radiation source operating on a mixture of argon with chlorine molecules|journal=Technical Physics Letters|date=5 February 2011|volume=34|issue=1|pages=14–16|doi=10.1134/S1063785008010057|language=en|issn=1063-7850|bibcode=2008TePhL..34...14S|s2cid=120130546}}</ref>

===Argon clusters===
Cooled argon gas can form clusters of atoms. [[Diargon]], also known as the argon dimer, has a binding energy of 0.012&nbsp;eV, but the Ar<sub>13</sub> and Ar<sub>19</sub> clusters have a [[Sublimation (phase transition)|sublimation]] energy (per atom) of 0.06&nbsp;eV. For liquid argon, which could be written as Ar<sub>∞</sub>, the energy increases to 0.08&nbsp;eV. Clusters of up to several hundred argon atoms have been detected. These argon clusters are [[icosahedral]] in shape, consisting of shells of atoms arranged around a central atom. The structure changes for clusters with more than 800 atoms to resemble a tiny crystal with a [[face-centered cubic]] (fcc) structure, as in solid argon. It is the surface energy that maintains an [[icosahedron|icosahedral]] shape, but for larger clusters internal pressure will attract the atoms into an fcc arrangement.<ref name=john02>{{cite book|last1=Johnston|first1=Roy L.|title=Atomic and Molecular Clusters|date=2002|publisher=Taylor & Francis|location=London|isbn=978-0-7484-0930-3|pages=50–62}}</ref> Neutral argon clusters are transparent to visible light.<ref name=john02/>

===Diatomic van der Waals molecules===
:{|class="wikitable"
!Molecule
!Binding energy<br>ground Σ state<br>(cm<sup>−1</sup>)
!Binding energy<br>excited Π state<br>(cm<sup>−1</sup>)
!Ground state<br>bond length<br>(Å)
!Excited state<br>bond length<br>(Å)
!CAS number<ref>{{Cite book|last1=Hüttner|first1=W.|title=Diamagnetic Diatomic Molecules. Part 1  |journal=Landolt Börnstein |volume=29A1|doi=10.1007/978-3-540-69954-5|series=Landolt-Börnstein – Group II Molecules and Radicals|year=2012|isbn=978-3-540-69953-8|bibcode=2012LanB.29A1.....H}}</ref>
|-
|ArH
|
|
|
|
|30736-04-0
|-
|ArHe
|
|
|
|
|12254-69-2
|-
|LiAr
|42.5
|925
|4.89
|2.48<ref>{{cite journal|last1=Brühl|first1=Rüdiger|last2=Zimmermann|first2=Dieter|title=High-resolution laser spectroscopy of the A ← X transition of LiAr|journal=Chemical Physics Letters|date=February 1995|volume=233|issue=4|pages=455–460|doi=10.1016/0009-2614(94)01476-C|bibcode=1995CPL...233..455B}}</ref>
|
|-
|BAr
|
|
|
|
|149358-32-7
|-
|ArNe
|
|
|
|
|12301-65-4
|-
|NaAr
|40
|560
|
|
|56633-38-6
|-
|[[MgAr]]
|44
|246
|
|
|72052-59-6
|-
|AlAr
|
|
|
|
|143752-09-4
|-
|SiAr<ref>{{cite journal|last1=Tao|first1=Chong|last2=Teslja|first2=Alexey|last3=Dagdigian|first3=Paul J.|last4=Atahan|first4=Sule|last5=Alexander|first5=Millard H.|title=Laser spectroscopic study of the SiAr van der Waals complex|journal=The Journal of Chemical Physics|date=2002|volume=116|issue=21|pages=9239|doi=10.1063/1.1473814|bibcode=2002JChPh.116.9239T}}</ref>
|
|
|
|
|
|-
|ArCl
|
|
|
|
|54635-29-9
|-
|Ar<sub>2</sub>
|
|
|
|
|12595-59-4
|-
|KAr
|42
|373
|
|
|12446-47-8
|-
|CaAr
|62
|134
|
|
|72052-60-9
|-
|SrAr
|68
|136
|
|
|
|-
|NiAr
|
|
|
|
|401838-48-0
|-
|ZnAr
|96
|706
|
|
|72052-61-0
|-
|GaAr
|
|
|
|
|149690-22-2
|-
|GeAr<ref name="ReferenceB"/>
|
|
|
|
|
|-
|KrAr
|
|
|
|
|51184-77-1
|-
|AgAr
|90
|1200
|
|
|
|-
|CdAr<ref name=cdar>{{cite journal|last1=Bennett|first1=Robert R.|last2=Breckenridge|first2=W. H.|title=Van der Waals bonding in the lowest electronic states of MgAr, ZnAr, CdAr, and HgAr: Spectroscopic characterization of the b3Π2 and e3Σ+ states of the CdAr molecule|journal=The Journal of Chemical Physics|date=15 January 1992|volume=96|issue=2|pages=882–890|doi=10.1063/1.462108|bibcode=1992JChPh..96..882B}}</ref>
|106
|544
|
|
|72052-62-1
|-
|InAr<ref>{{cite journal|last1=Fawzy|first1=Wafaa M.|last2=Le Roy|first2=Robert J.|last3=Simard|first3=Benoit|last4=Niki|first4=Hideaki|last5=Hackett|first5=Peter A.|s2cid=95444756|title=Determining repulsive potentials of InAr from oscillatory bound→continuum emission|journal=The Journal of Chemical Physics|date=1993|volume=98|issue=1|pages=140|doi=10.1063/1.464663|bibcode=1993JChPh..98..140F}}</ref>
|
|
|
|
|146021-90-1
|-
|SnAr<ref name="ReferenceC"/>
|
|
|
|
|
|-
|ArXe
|
|
|
|
|58206-67-0
|-
|AuAr
|
|
|
|
|195245-92-2
|-
|HgAr
|131
|446
|
|
|87193-95-1
|}

ArO* is also formed when [[dioxygen]] trapped in an argon matrix is subjected to [[vacuum ultraviolet]]. It can be detected by its luminescence:
:O<sub>2</sub> + ''hv'' → {{chem|O|2|+}} + e<sup>−</sup>;{{pad|2em}} {{chem|O|2|+}} + e<sup>−</sup> → 2O*;{{pad|2em}} O* + Ar → ArO*.<ref>{{cite journal|last1=Laarmann|first1=T.|last2=Wabnitz|first2=H.|last3=von Haeften|first3=K.|last4=Möller|first4=T.|title=Photochemical processes in doped argon-neon core-shell clusters: The effect of cage size on the dissociation of molecular oxygen|journal=The Journal of Chemical Physics|date=2008|volume=128|issue=1|pages=014502|doi=10.1063/1.2815798|pmid=18190199|bibcode=2008JChPh.128a4502L|url=http://bib-pubdb1.desy.de//record/86124/files/GetPDFServlet.pdf}}</ref>
Light emitted by ArO* has two main bands, one at 2.215&nbsp;eV, and a weaker one at 2.195&nbsp;eV.<ref>{{cite journal|last1=Taylor|first1=R. V.|last2=Walker|first2=W. C.|title=Photoluminescence of ArO and KrO in doped rare-gas matrices|journal=The Journal of Chemical Physics|date=1979|volume=70|issue=1|pages=284|doi=10.1063/1.437161|bibcode=1979JChPh..70..284T}}</ref>

Argon sulfide, ArS* luminesces in the [[near infrared]] at 1.62&nbsp;eV. ArS is made from UV irradiated [[carbonyl sulfide|OCS]] in an argon matrix. The excited states lasts for 7.4 and 3.5&nbsp;μs for spectrum peak and band respectively.<ref>{{cite journal|last1=Taylor|first1=R. V.|last2=Walker|first2=W. C.|title=Photoluminescence of ArS, KrS, and XeS in rare-gas matrices|journal=Applied Physics Letters|date=1979|volume=35|issue=5|pages=359|doi=10.1063/1.91149|bibcode=1979ApPhL..35..359T}}</ref>

===Triatomic van der Waals molecules===
Cluster molecules containing [[dichlorine]] and more than one argon atom can be made by forcing a 95:5 mixture of helium and argon and a trace of chlorine though a nozzle. ArCl<sub>2</sub> exists in a T shape. Ar<sub>2</sub>Cl<sub>2</sub> has a distorted tetrahedron shape, with the two argon atoms 4.1&nbsp;Å from each other, and their axis 3.9&nbsp;Å from the Cl<sub>2</sub>. The van der Waals bond energy is 447&nbsp;cm<sup>−1</sup>. Ar<sub>3</sub>Cl<sub>2</sub> also exists with a van der Waals bond energy of 776&nbsp;cm<sup>−1</sup>.<ref>{{cite journal|last1=Bieler|first1=Craig R.|last2=Evard|first2=Dwight D.|last3=Janda|first3=Kenneth C.|title=Argon chloride (Ar<sub>2</sub>Cl<sub>2</sub> and Ar<sub>3</sub>Cl<sub>2</sub>): structure, bond energy, and dissociation dynamics|journal=The Journal of Physical Chemistry|date=September 1990|volume=94|issue=19|pages=7452–7457|doi=10.1021/j100382a027}}</ref>

The linear Ar·Br<sub>2</sub> molecule has a continuous spectrum for [[bromine]] molecule X&nbsp;→&nbsp;B transitions. The spectrum of bromine is blue-shifted and spread out when it binds an argon atom.<ref name="pio"/>

ArI<sub>2</sub> shows a spectrum that adds satellite bands to the higher vibrational bands of [[iodine|I<sub>2</sub>]].<ref>{{cite journal|last1=Kubiak|first1=Glenn|last2=Fitch|first2=Pamela S. H.|last3=Wharton|first3=Lennard|last4=Levy|first4=Donald H.|title=The fluorescence excitation spectrum of the ArI<sub>2</sub> van der Waals complex|journal=The Journal of Chemical Physics|date=1978|volume=68|issue=10|pages=4477|doi=10.1063/1.435530|bibcode=1978JChPh..68.4477K|doi-access=free}}</ref> The ArI<sub>2</sub> molecule has two different isomers, one shape is linear, and the other is T-shaped. The dynamics of ArI<sub>2</sub> is complex. Breakup occurs through different routes in the two isomers. The T shape undergoes intramolecular vibrational relaxation, whereas the linear one directly breaks apart.<ref name="rohrb">{{cite journal|last1=Rohrbacher|first1=Andreas|last2=Halberstadt|first2=Nadine|last3=Janda|first3=Kenneth C.|title=The Dynamics of Noble Gas – Halogen Molecules and Clusters|journal=Annual Review of Physical Chemistry|date=October 2000|volume=51|issue=1|pages=405–433|doi=10.1146/annurev.physchem.51.1.405|pmid=11031288|bibcode=2000ARPC...51..405R}}</ref> Diiodine clusters, I<sub>2</sub>Ar<sub>''n''</sub> have been made.<ref>{{cite journal|last1=Fei|first1=Suli|last2=Zheng|first2=Xiaonan|last3=Heaven|first3=Michael C.|last4=Tellinghuisen|first4=Joel|title=Spectroscopy and relaxation dynamics of I<sub>2</sub>Ar<sub>''n''</sub> clusters. Geminate recombination and cluster fragmentation|journal=The Journal of Chemical Physics|date=1992|volume=97|issue=9|pages=6057|doi=10.1063/1.463716|bibcode=1992JChPh..97.6057F|doi-access=free}}</ref>

The ArClF cluster has a linear shape.<ref>{{cite journal|last1=Harris|first1=Stephen J.|title=Intermolecular potential between an atom and a diatomic molecule: The structure of ArCIF|journal=The Journal of Chemical Physics|date=1974|volume=61|issue=1|pages=193–197|doi=10.1063/1.1681622|bibcode=1974JChPh..61..193H}}</ref> The argon atom is closest to the chlorine atom.<ref name="pio">{{cite journal|last1=Pio|first1=Jordan M.|last2=van der Veer|first2=Wytze E.|last3=Bieler|first3=Craig R.|last4=Janda|first4=Kenneth C.|s2cid=21324093|title=Product state resolved excitation spectroscopy of He–, Ne–, and Ar–Br<sub>2</sub> linear isomers: Experiment and theory|journal=The Journal of Chemical Physics|date=2008|volume=128|issue=13|pages=134311|doi=10.1063/1.2885047|pmid=18397068|bibcode=2008JChPh.128m4311P}}</ref>

Linear ArBrCl can also rearrange to ArClBr, or a T-shaped isomer.<ref>{{cite journal|last1=Zheng|first1=Rui|last2=Li|first2=Song|last3=Chen|first3=Shan-Jun|last4=Chen|first4=Yan|last5=Zheng|first5=Li-Min|title=Theoretical investigation of potential energy surface and bound states for the van der Waals complex Ar–BrCl dimer|journal=Chemical Physics|date=September 2015|volume=458|pages=77–85|doi=10.1016/j.chemphys.2015.07.013|bibcode=2015CP....458...77Z}}</ref>

Multiple argon atoms can "[[solvation|solvate]]" a [[water]] molecule forming a monolayer around the H<sub>2</sub>O. Ar<sub>12</sub>·H<sub>2</sub>O is particularly stable, having an [[icosahedral]] shape. Molecules from Ar·H<sub>2</sub>O to Ar<sub>14</sub>·H<sub>2</sub>O have been studied.<ref>{{cite journal|last1=Liu|first1=Suyan|last2=Bačić|first2=Zlatko|last3=Moskowitz|first3=Jules W.|last4=Schmidt|first4=Kevin E.|title=Ar<sub>''n''</sub>H<sub>2</sub>O (''n'' = 1–14) van der Waals clusters: Size evolution of equilibrium structures|journal=The Journal of Chemical Physics|date=1994|volume=101|issue=10|pages=8310|doi=10.1063/1.468097|bibcode=1994JChPh.101.8310L}}</ref>

ArBH was produced from [[boron monohydride]] (BH) which in turn was created from [[diborane]] by way of an ultraviolet 193&nbsp;nm laser. The BH-argon mixture was expanded through a 0.2&nbsp;mm diameter nozzle into a vacuum. The gas mixture cools and Ar and BH combine to yield ArBH. A band spectrum that combines the A<sup>1</sup>Π←X<sup>1</sup>Σ<sup>+</sup> electronic transition, with vibration and rotation can be observed. The BH has singlet spin, and this is the first known van der Waals complex with a singlet spin pair of atoms. For this molecule the rotational constant is 0.133&nbsp;cm<sup>−1</sup>, The dissociation energy is 92&nbsp;cm<sup>−1</sup> and distance from argon to boron atom is 3.70&nbsp;Å.<ref>{{cite journal|last1=Hwang|first1=Eunsook|last2=Dagdigian|first2=Paul J.|title=Observation and characterization of the ArBH(X1Σ+,A1Π) van der Waals complex through fluorescence excitation spectroscopy|journal=The Journal of Chemical Physics|date=15 August 1994|volume=101|issue=4|pages=2903–2913|doi=10.1063/1.467603|bibcode=1994JChPh.101.2903H}}</ref> ArAlH is also known to exist.<ref>{{cite journal|last1=Hwang|first1=Eunsook|last2=Dagdigian|first2=Paul J.|title=Fluorescence excitation spectroscopy and dynamics of the ArAlH(X 1Σ+,A 1Π) van der Waals complex|journal=The Journal of Chemical Physics|date=8 February 1995|volume=102|issue=6|pages=2426–2439|doi=10.1063/1.468673|issn=0021-9606|bibcode=1995JChPh.102.2426H}}</ref>

MgAr<sub>2</sub> is also known.<ref name=cdar/>

===Polyatomic van der Waals molecules===
Some linear polyatomic molecules can form T-shaped van der Waals complexes with argon.  These include [[Cyanogen|NCCN]], [[carbon dioxide]], [[nitrous oxide]], [[acetylene]], [[carbon oxysulfide]], and [[cyanogen chloride|ClCN]].  Others attach the argon atom at one end to continue to be linear, including [[hydrogen cyanide|HCN]].<ref name=eben/>

Other polyatomic van der Waals compounds of argon, include those of [[fluorobenzene]],<ref>{{cite journal|last1=Gascooke|first1=Jason R.|last2=Alexander|first2=Ula N.|last3=Lawrance|first3=Warren D.|title=Two-dimensional laser induced fluorescence spectroscopy of van der Waals complexes: Fluorobenzene-Ar<sub>''n''</sub> (''n'' = 1,2)|journal=The Journal of Chemical Physics|date=2012|volume=136|issue=13|pages=134309|doi=10.1063/1.3697474|pmid=22482554|bibcode=2012JChPh.136m4309G|hdl=2328/26538|hdl-access=free}}</ref> formyl radical (ArHCO),<ref>{{cite journal|last1=Wright|first1=Scott A.|last2=Dagdigian|first2=Paul J.|title=Fluorescence excitation spectroscopy of the Ar–HCO(X̃<sup>2</sup>Aʹ,B̃<sup>2</sup>Aʹ) van der Waals complex|journal=The Journal of Chemical Physics|date=1997|volume=107|issue=3|pages=680|doi=10.1063/1.474469|bibcode=1997JChPh.107..680W|doi-access=free}}</ref> [[7-azaindole]],<ref>{{cite journal|last1=Kang|first1=Cheolhwa|last2=Yi|first2=John T.|last3=Pratt|first3=David W.|title=High resolution electronic spectra of 7-azaindole and its Ar atom van der Waals complex|journal=The Journal of Chemical Physics|date=2005|volume=123|issue=9|pages=094306|doi=10.1063/1.1990119|pmid=16164344|bibcode=2005JChPh.123i4306K}}</ref> [[glyoxal]],<ref>{{cite journal|last1=Lapierre|first1=Luc|last2=Frye|first2=Donald|last3=Dai|first3=Hai-Lung|title=Isomeric structures and van der Waals vibrational frequencies of the glyoxal·Ar complexes. I. Fluorescence excitation spectroscopy|journal=The Journal of Chemical Physics|date=1992|volume=96|issue=4|pages=2703|doi=10.1063/1.462018|bibcode=1992JChPh..96.2703L}}</ref> sodium chloride (ArNaCl),<ref>{{cite journal|last1=Mizoguchi|first1=Asao|last2=Endo|first2=Yasuki|last3=Ohshima|first3=Yasuhiro|title=Rotational spectrum of a salt-containing van der Waals complex: Ar–NaCl|journal=The Journal of Chemical Physics|date=1998|volume=109|issue=24|pages=10539|doi=10.1063/1.477754|bibcode=1998JChPh.10910539M}}</ref> ArHCl,<ref>{{cite journal|last1=Girardet|first1=C.|last2=Lakhlifi|first2=A.|last3=Laroui|first3=B.|title=Infrared profile of van der Waals dimers HCl–RG* (RG*=Ar, Kr, Xe) trapped in rare gas matrices|journal=The Journal of Chemical Physics|date=1992|volume=97|issue=11|pages=7955|doi=10.1063/1.463470|bibcode=1992JChPh..97.7955G}}</ref> and [[cyclopentanone]].<ref>{{cite journal|last1=Lin|first1=Wei|last2=Brooks|first2=Andrew H.|last3=Minei|first3=Andrea J.|last4=Novick|first4=Stewart E.|last5=Pringle|first5=Wallace C.|title=Microwave Spectra and Structure of the Argon–Cyclopentanone and Neon–Cyclopentanone van der Waals Complexes|journal=The Journal of Physical Chemistry A|date=6 February 2014|volume=118|issue=5|pages=856–861|doi=10.1021/jp410381r|pmid=24428820|bibcode=2014JPCA..118..856L}}</ref>

:{|class="wikitable"
!Molecule
!Name
!Ground state<br>binding energy<br>(cm<sup>−1</sup>)
!Closest position or atom<br>to argon
!Ground state<br>bond length of Ar<br>(Å)
!Bond angle<br>from atom<br>(degrees)
!Bond stretch force<br/> or frequency
!dipole moment D
!CAS number
!references
|-
|(CH<sub>3</sub>)<sub>2</sub>F<sub>2</sub>Si·Ar
| [[Difluorodimethylsilane]] – argon
|
|
|
|
|
|
|
|
|-
|CH<sub>2</sub>F<sub>2</sub>·Ar
|[[Difluoromethane]] – argon
|
|F
|3.485
|58.6
|
|
|
| <ref>{{Citation |last=Vogt |first=J. |title=58 CH2ArF2 Difluoromethane - argon (1/1) |date=2010 |url=http://materials.springer.com/lb/docs/sm_lbs_978-3-642-10371-1_60 |work=Asymmetric Top Molecules. Part 1 |series=Landolt-Börnstein - Group II Molecules and Radicals |volume=29D1 |pages=131–132 |editor-last=Hüttner |editor-first=W. |access-date=2024-01-08 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-642-10371-1_60 |isbn=978-3-642-10370-4}}</ref>
|-
|CF<sub>3</sub>CN
|[[trifluoromethylcyanide]] argon
|
|C1
|3.73
|77
|
|
|947504-98-5
|<ref>{{Citation |last=Vogt |first=J. |title=CF3CN Trifluoromethylcyanide - argon (1/1) |date=2010 |url= |work=Asymmetric Top Molecules. Part 1 |volume=29D1 |pages= |editor-last=Hüttner |editor-first=W. |access-date= |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi= |isbn=978-3-642-10370-4}}</ref>
|-
||CF<sub>2</sub>HCH<sub>3</sub>·Ar
|[[1,1-difluoroethane]] argon
|
|F
|
|
|
|
| –
|<ref name="Demaison2010">{{cite book|last1=Demaison|first1=J.|chapter=172 C2H4ArF2 1,1-Difluoroethane - argon (1/1) |title=Asymmetric Top Molecules. Part 1|volume=29D1|year=2010|pages=351|doi=10.1007/978-3-642-10371-1_174|series=Landolt-Börnstein – Group II Molecules and Radicals|isbn=978-3-642-10370-4|bibcode=2010LanB.29D1..351D}}</ref>
|-
|CH<sub>2</sub>FCH<sub>2</sub>F·Ar
|[[1,2-difluoroethane]] argon
|181
|F
|3.576
|61
|
|
|264131-14-8
|<ref>{{cite journal|last1=Melandri|first1=Sonia|last2=Velino|first2=Biagio|last3=Favero|first3=Paolo G|last4=Dell'Erba|first4=Adele|last5=Caminati|first5=Walther|title=Investigation of a van der Waals complex with ''C''<sub>1</sub> symmetry: the free-jet rotational spectrum of 1,2-difluoroethane–Ar|journal=Chemical Physics Letters|date=April 2000|volume=321|issue=1–2|pages=31–36|doi=10.1016/S0009-2614(00)00326-2|bibcode=2000CPL...321...31M}}</ref>
|-
|CH<sub>3</sub>CHO·Ar
|[[Acetaldehyde]] argon
|161
|C-1
|3.567
|76.34
|
|
|158885-13-3
|<ref>{{cite journal|last1=Melandri|first1=Sonia|last2=Dell'Erba|first2=Adele|last3=Favero|first3=Paolo G.|last4=Caminati|first4=Walther|title=Millimeter-wave investigation, simplified interpretation of the fourfold rotational spectrum, and dynamics of the internal motions of acetaldehyde–argon|journal=Journal of Molecular Spectroscopy|date=November 2003|volume=222|issue=1|pages=121–128|doi=10.1016/S0022-2852(03)00197-8|bibcode=2003JMoSp.222..121M}}</ref>
|-
|C<sub>2</sub>H<sub>4</sub>O·Ar
|[[oxirane]] argon
|200
|O
|3.606 (CM)
|72.34
|
|
|
|<ref>{{cite journal|last1=Moreschini|first1=Paolo|last2=Caminati|first2=Walther|last3=Favero|first3=Paolo G.|last4=Legon|first4=Anthony C.|title=Pathways for inversion in the oxirane–argon complex|journal=Journal of Molecular Structure|date=December 2001|volume=599|issue=1–3|pages=81–87|doi=10.1016/S0022-2860(01)00846-8|bibcode=2001JMoSt.599...81M}}</ref>
|-
|ArBF<sub>3</sub>
|[[Boron trifluoride]] argon
|
|B
|3.325
|on axis ArBF ≈90.5°
|0.030 mdyn/Å
|0.176
|
|<ref>{{cite journal|last1=Janda|first1=K. C.|last2=Bernstein|first2=L. S.|last3=Steed|first3=J. M.|last4=Novick|first4=S. E.|last5=Klemperer|first5=W.|title=Synthesis, microwave spectrum, and structure of ArBF3, BF3CO, and N2BF3|journal=Journal of the American Chemical Society|date=December 1978|volume=100|issue=26|pages=8074–8079|doi=10.1021/ja00494a008}}</ref>
|-
|ArC<sub>6</sub>H<sub>6</sub>
|[[benzene]]-argon
|
|on sixfold axis
|3.53 from plane
|
|
|0.12
|
|<ref>{{cite journal|last1=Brupbacher|first1=Th.|last2=Bauder|first2=A.|title=Rotational spectrum and dipole moment of the benzene—argon van der Waals complex|journal=Chemical Physics Letters|date=October 1990|volume=173|issue=5–6|pages=435–438|doi=10.1016/0009-2614(90)87230-O|bibcode=1990CPL...173..435B}}</ref>
|-
|ArPF<sub>3</sub>
|argon [[phosphorus trifluoride]] complex
|
|P
|3.953 from centre of mass
|70.3° on PF<sub>2</sub> face
|
|
|
|<ref>{{cite journal|last1=Hillig|first1=Kurt W.|last2=Matos|first2=Jose|last3=Scioly|first3=Anthony|last4=Kuczkowski|first4=Robert L.|title=The microwave spectrum of argon-phosphorus trifluoride|journal=Chemical Physics Letters|date=January 1987|volume=133|issue=4|pages=359–362|doi=10.1016/0009-2614(87)87082-3|bibcode=1987CPL...133..359H|hdl=2027.42/26836|hdl-access=free}}</ref>
|-
|Ar-NCCN
|argon–[[cyanogen]] van der Waals complex
|
|centre of molecule
|3.58
|90° T shape
|30&nbsp;cm<sup>−1</sup>
|0.0979
|
|<ref name=eben>{{cite journal|last1=Ebenstein|first1=William L.|last2=Muenter|first2=J. S.|title=Structure and properties of the argon–cyanogen van der Waals complex a)|journal=The Journal of Chemical Physics|date=1984|volume=80|issue=4|pages=1417|doi=10.1063/1.446877|bibcode=1984JChPh..80.1417E}}</ref>
|-
|DCCDAr
|argon-deuterated acetylene
|
|centre of molecule
|3.25
|90° T shape
|0.0008 mdyn/Å / 8.7&nbsp;cm<sup>−1</sup>
|
|
|<ref name=eben/>
|-
|SO<sub>3</sub>Ar
|sulfur trioxide argon
|
|S
|3.350
|on axis 90° from SO bond
|0.059&nbsp;mdyn/Å / 61&nbsp;cm<sup>−1</sup>
|
|
|<ref>{{cite journal|last1=Bowen|first1=K. H.|last2=Leopold|first2=K. R.|last3=Chance|first3=K. V.|last4=Klemperer|first4=W.|title=Weakly bound complexes of sulfur trioxide: The structure of ArSO3 and the dipole moment of N2SO3|journal=The Journal of Chemical Physics|date=1980|volume=73|issue=1|pages=137|doi=10.1063/1.439908|bibcode=1980JChPh..73..137B}}</ref>
|-
|Ar•HCCH
|acetylene argon
|
|
|
|T shape
|
|
|
|<ref name="dson">{{cite book|editor1=G. Davidson|editor2=E. A. V.Ebsworth|title=Spectroscopic Properties of Inorganic and Organometallic Compounds|date=2007|publisher=Royal Society of Chemistry|page=200|isbn=9781847555076|chapter-url=https://books.google.com/books?id=3XUoDwAAQBAJ&pg=200|language=en|chapter=Rotational Spectroscopy|first1=J. H.|last1=Carpenter}}</ref>
|-
|OCS•Ar|| || || || || || || || ||<ref name="dson"/>
|-
|CH<sub>3</sub>OH•Ar|| || || || || || || || ||<ref name="dson"/>
|-
|CH<sub>3</sub>Cl•Ar|| || || || || || || || ||<ref name="dson"/>
|-
|
|Pyridine argon|| || || || || || || ||<ref name="dson"/>
|-
|
|Pyrrole argon|| || || || || || || ||<ref name="dson"/>
|}

==Aqueous argon==
Argon dissolved in water causes the [[pH]] to rise to 8.0,<ref>{{cite journal|last1=Holleran|first1=Eugene M.|last2=Hennessy|first2=John T.|last3=LaPietra|first3=Frank R.|title=Anomalous pH changes in solutions of inert gases possibly indicating their basicity|journal=The Journal of Physical Chemistry|date=August 1967|volume=71|issue=9|pages=3081–3084|doi=10.1021/j100868a060}}</ref> apparently by reducing the number of oxygen atoms available to bind protons.<ref>{{cite journal|last1=Tadros|first1=T. F.|last2=Sadek|first2=H.|title=Medium effects in aqueous non-electrolyte solutions|journal=Electrochimica Acta|date=May 1969|volume=14|issue=5|pages=377–388|doi=10.1016/0013-4686(69)80014-9}}</ref>

With ice, argon forms a [[clathrate hydrate]]. Up to 0.6&nbsp;GPa, the clathrate has a cubic structure. Between 0.7 and 1.1&nbsp;GPa the clathrate has a tetragonal structure. Between 1.1 and 6.0&nbsp;GPa the structure is [[body centered]] orthorhombic. Over 6.1&nbsp;GPa, the clathrate converts into solid argon and [[ice VII]].<ref>{{cite journal|last1=Hirai|first1=Hisako|last2=Uchihara|first2=Yukako|last3=Nishimura|first3=Yukiko|last4=Kawamura|first4=Taro|last5=Yamamoto|first5=Yoshitaka|last6=Yagi|first6=Takehiko|title=Structural Changes of Argon Hydrate under High Pressure|journal=The Journal of Physical Chemistry B|date=October 2002|volume=106|issue=43|pages=11089–11092|doi=10.1021/jp021458l}}</ref> At atmospheric pressure the clathrate is stable below 147&nbsp;K.<ref>{{cite journal|last1=Yang|first1=L.|last2=Tulk|first2=C. A.|last3=Klug|first3=D. D.|last4=Chakoumakos|first4=B. C.|last5=Ehm|first5=L.|last6=Molaison|first6=J. J.|last7=Parise|first7=J. B.|last8=Simonson|first8=J. M.|title=Guest disorder and high pressure behavior of argon hydrates|journal=Chemical Physics Letters|date=January 2010|volume=485|issue=1–3|pages=104–109|doi=10.1016/j.cplett.2009.12.024|bibcode=2010CPL...485..104Y|s2cid=93956695 |url=https://nrc-publications.canada.ca/eng/view/accepted/?id=6422fe3a-4705-4b9d-8806-07f15c5e76fa}}</ref> At 295&nbsp;K the argon pressure from the clathrate is 108&nbsp;MPa.<ref>{{cite journal|last1=Sugahara|first1=Keisuke|last2=Kaneko|first2=Ryuji|last3=Sasatani|first3=Arata|last4=Sugahara|first4=Takeshi|last5=Ohgaki|first5=Kazunari|title=Thermodynamic and Raman Spectroscopic Studies of Ar Hydrate System|journal=The Open Thermodynamics Journal|date=30 October 2008|volume=2|issue=1|pages=95–99|doi=10.2174/1874396X00802010095|doi-access=free}}</ref>

==Argon fluorohydride==
[[Argon fluorohydride]] was an important discovery in the rejuvenation of the study of noble gas chemistry. HArF is stable in solid form at temperatures below 17&nbsp;K.<ref>{{cite journal|last1=Räsänen|first1=Markku|last2=Khriachtchev|first2=Leonid|last3=Pettersson|first3=Mika|last4=Runeberg|first4=Nino|last5=Lundell|first5=Jan|title=A stable argon compound|journal=Nature|date=24 August 2000|volume=406|issue=6798|pages=874–876|doi=10.1038/35022551|pmid=10972285|bibcode=2000Natur.406..874K|s2cid=4382128}}</ref> It is prepared by photolysis of [[hydrogen fluoride]] in a solid argon matrix.<ref name="mcd">{{cite journal|last1=McDowell|first1=Sean|title=Studies of Neutral Rare-Gas Compounds and their Non-Covalent Interactions with Other Molecules|journal=Current Organic Chemistry|date=1 May 2006|volume=10|issue=7|pages=791–803|doi=10.2174/138527206776818964}}</ref> HArArF would have such a low barrier to decomposition that it will likely never be observed.<ref>{{cite journal|last1=Jiménez Halla|first1=C. Óscar C.|last2=Fernández|first2=Israel|last3=Frenking|first3=Gernot|title=Is it Possible To Synthesize a Neutral Noble Gas Compound Containing a Ng–Ng Bond? A Theoretical Study of H–Ng–Ng–F (Ng=Ar, Kr, Xe)|journal=Angewandte Chemie International Edition|date=2 January 2009|volume=48|issue=2|pages=366–369|doi=10.1002/anie.200803252|pmid=19053096}}</ref> However HBeArF is predicted to be more stable than HArF.<ref>{{cite journal|last1=Jayasekharan|first1=T.|last2=Ghanty|first2=Tapan K.|title=Significant increase in the stability of rare gas hydrides on insertion of beryllium atom|journal=The Journal of Chemical Physics|date=2007|volume=127|issue=11|pages=114314|doi=10.1063/1.2768936|pmid=17887844|bibcode=2007JChPh.127k4314J}}</ref>

==Uranium compounds==
CUO in a solid argon matrix can bind one, or a few argon atoms to yield CUO·Ar, CUO·Ar<sub>3</sub> or CUO·Ar<sub>4</sub>. CUO itself is made by evaporating [[uranium]] atoms into [[carbon monoxide]]. Uranium acts as a strong [[Lewis acid]] in CUO<ref name="mcd"/><ref name="liang002">{{cite journal|last1=Liang|first1=Binyong|last2=Andrews|first2=Lester|last3=Li|first3=Jun|last4=Bursten|first4=Bruce E.|title=Noble Gas−Actinide Compounds: Evidence for the Formation of Distinct CUO(Ar)<sub>4−''n''</sub>(Xe)<sub>n</sub> and CUO(Ar)<sub>4−''n''</sub>(Kr)<sub>''n''</sub> (''n'' = 1, 2, 3, 4) Complexes|journal=Journal of the American Chemical Society|date=August 2002|volume=124|issue=31|pages=9016–9017|doi=10.1021/ja026432m|pmid=12148982}}</ref> and forms bonds with energies of about 3.2&nbsp;kcal/mol (13.4&nbsp;kJ/mol) with argon. The argon acts as a [[Lewis base]]. Its electron density is inserted into an empty 6d orbital on the uranium atom. The spectrum of CUO is changed by argon so that the U−O stretch frequency changes from 872.2 to 804.3&nbsp;cm<sup>−1</sup> and the U−C stretch frequency from 1047.3 to 852.5&nbsp;cm<sup>−1</sup>.<ref name="li02"/> The significant change in the spectrum occurs because the CUO is changed from a singlet state (in gas phase or solid neon) to a triplet state, with argon or noble gas complexing.<ref name="andrews">{{cite journal|last1=Andrews|first1=Lester|last2=Liang|first2=Binyong|last3=Li|first3=Jun|last4=Bursten|first4=Bruce E.|title=Noble Gas−Actinide Complexes of the CUO Molecule with Multiple Ar, Kr, and Xe Atoms in Noble-Gas Matrices|journal=Journal of the American Chemical Society|date=March 2003|volume=125|issue=10|pages=3126–3139|doi=10.1021/ja027819s|pmid=12617681}}</ref> The argon–uranium bond length is 3.16&nbsp;Å.<ref name="li02">{{cite journal|last1=Li|first1=J.|title=Noble Gas–Actinide Compounds: Complexation of the CUO Molecule by Ar, Kr, and Xe Atoms in Noble Gas Matrices|journal=Science|date=28 February 2002|volume=295|issue=5563|pages=2242–2245|doi=10.1126/science.1069342|pmid=11872801|bibcode=2002Sci...295.2242L|s2cid=45390086}}</ref> This is shorter than the sum of atomic radii of U and Ar of 3.25&nbsp;Å, but considerably longer than a normal covalent bond to uranium. For example, U−Cl in [[uranium hexachloride|UCl<sub>6</sub>]] is 2.49&nbsp;Å.<ref name="andrews"/> When [[xenon]] is included in the solid argon matrix up to a few percent, additional van der Waals molecules are formed: CUO·Ar<sub>3</sub>Xe, CUO·Ar<sub>2</sub>Xe<sub>2</sub>, CUO·ArXe<sub>3</sub> and CUO·Xe<sub>4</sub>.<ref name="liang002"/> Similarly krypton can substitute for argon in CUO·Ar<sub>3</sub>Kr, CUO·Ar<sub>2</sub>Kr<sub>2</sub>, CUO·ArKr<sub>3</sub> and CUO·Kr<sub>4</sub>.<ref name="andrews"/> The shape of these molecules is roughly [[octahedral]], with a uranium centre and with the noble gas atoms around the equator.<ref name="andrews"/>

[[Uranyl(V)|{{chem|UO|2|+}}]] can bind up to five noble gas atoms in a ring around a linear O={{overset|+|U}}=O core.<ref name=wang04>{{cite journal|last1=Wang|first1=Xuefeng|last2=Andrews|first2=Lester|last3=Li|first3=Jun|last4=Bursten|first4=Bruce E.|title=Significant Interactions between Uranium and Noble-Gas Atoms: Coordination of the {{chem|UO|2|+}} Cation by Ne, Ar, Kr, and Xe Atoms|journal=Angewandte Chemie International Edition|date=3 May 2004|volume=43|issue=19|pages=2554–2557|doi=10.1002/anie.200453790|pmid=15127451}}</ref> These molecules are produced when uranium metal is laser ablated into dioxygen. This produces UO, UO<sub>2</sub>, UO<sub>3</sub>, U<sup>+</sup>, and importantly {{chem|UO|2|+}}. {{chem|UO|2|+}} is then condensed into a noble gas matrix, either a pure element or a mixture. Heavier noble gas atoms will tend to displace the lighter atoms. Ionic molecules produced this way include {{chem|link=Uranyl Tetraneoargate|UO|2|Ne|4|Ar|+}}, {{chem|UO|2|Ne|3|Ar|2|+}}, {{chem|UO|2|Ne|2|Ar|3|+}}, {{chem|UO|2|NeAr|4|+}}, {{chem|UO|2|Ar|5|+}}, {{chem|link=Uranyl Tetraargokryptate|UO|2|Ar|4|Kr|+}}, {{chem|UO|2|Ar|3|Kr|2|+}}, {{chem|UO|2|Ar|2|Kr|3|+}}, {{chem|UO|2|ArKr|4|+}}, {{chem|UO|2|Ar|4|Xe|+}}, {{chem|UO|2|Ar|3|Xe|2|+}}, {{chem|UO|2|Ar|2|Xe|3|+}}, and {{chem|UO|2|ArXe|4|+}}, which are identified by a shift in the U=O antisymmetric stretching frequency.<ref name=wang04/>

Neutral [[uranium dioxide|UO<sub>2</sub>]] condensed in solid argon is converted from one electronic state to another by the argon atom ligands. In argon the electron configuration is 5f<sup>2</sup>(δφ) whereas in neon it is 5f<sup>1</sup>7s<sup>1</sup> (the state <sup>3</sup>H<sub>4g</sub> compared to <sup>3</sup>Φ<sub>2u</sub>). This is because the argon atoms have a larger antibonding interaction with the 7s<sup>1</sup> electron, forcing it into a different subshell. The argonated compound has a stretching frequency of 776&nbsp;cm<sup>−1</sup> compared to 914.8&nbsp;cm<sup>−1</sup> in [[neon]].<ref>{{cite journal|last1=Denning|first1=Robert G.|title=Electronic Structure and Bonding in Actinyl Ions and their Analogs|journal=The Journal of Physical Chemistry A|date=May 2007|volume=111|issue=20|pages=4125–4143|doi=10.1021/jp071061n|pmid=17461564|bibcode=2007JPCA..111.4125D}}</ref> The argon uranium dioxide molecule is likely UO<sub>2</sub>Ar<sub>5</sub>.<ref>{{cite journal|last1=Li|first1=Jun|last2=Bursten|first2=Bruce E.|last3=Andrews|first3=Lester|last4=Marsden|first4=Colin J.|title=On the Electronic Structure of Molecular UO<sub>2</sub> in the Presence of Ar Atoms: Evidence for Direct U−Ar Bonding|journal=Journal of the American Chemical Society|date=March 2004|volume=126|issue=11|pages=3424–3425|doi=10.1021/ja039933w|pmid=15025460}}</ref>

==Beryllium oxide==
When [[beryllium]] atoms react with oxygen in a solid argon matrix (or [[beryllia]] is evaporated into the matrix) ArBeO will be formed, and is observable by its infrared spectrum. The beryllia molecule is strongly polarised, and the argon atom is attracted to the beryllium atom.<ref name="andrews"/><ref>{{cite journal|last1=Thompson|first1=Craig A.|last2=Andrews|first2=Lester|title=Noble Gas Complexes with BeO: Infrared Spectra of NG-BeO (NG = Ar, Kr, Xe)|journal=Journal of the American Chemical Society|date=January 1994|volume=116|issue=1|pages=423–424|doi=10.1021/ja00080a069}}</ref> The bond strength of Ar−Be is calculated to be 6.7&nbsp;kcal/mol (28&nbsp;kJ/mol). The Ar−Be bond length is predicted to be 2.042&nbsp;Å.<ref>{{cite journal|last1=Veldkamp|first1=A.|last2=Frenking|first2=G.|title=Structures and bond energies of the noble gas complexes NgBeO (Ng=Ar, Kr, Xe)|journal=Chemical Physics Letters|date=August 1994|volume=226|issue=1–2|pages=11–16|doi=10.1016/0009-2614(94)00697-0|bibcode=1994CPL...226...11V}}</ref>

The cyclic Be<sub>2</sub>O<sub>2</sub> molecule can bind two argon atoms, or one argon along with another noble gas atom.<ref>{{cite book |last1=Grandinetti |first1=Felice |title=Noble Gas Chemistry: Structure, Bonding, and Gas-Phase Chemistry |date=2018 |publisher=John Wiley & Sons |isbn=9783527803545 |page=125 |url=https://books.google.com/books?id=3W5iDwAAQBAJ&pg=PT125 |language=en}}</ref>

Analogously, beryllium reacting with [[hydrogen sulfide]] and trapped in an argon matrix at 4&nbsp;K forms ArBeS. It has a binding energy calculated to be 12.8 kcal/mol (54&nbsp;kJ/mol).<ref>{{cite journal|last1=Wang|first1=Qiang|last2=Wang|first2=Xuefeng|title=Infrared Spectra of NgBeS (Ng = Ne, Ar, Kr, Xe) and BeS2 in Noble-Gas Matrices|journal=The Journal of Physical Chemistry A|date=21 February 2013|volume=117|issue=7|pages=1508–1513|doi=10.1021/jp311901a|pmid=23327099|bibcode=2013JPCA..117.1508W}}</ref>

ArBeO<sub>2</sub>CO (beryllium carbonate) has been prepared (along with Ne, Kr and Xe adducts).<ref>{{cite journal|last1=Zhang|first1=Qingnan|last2=Chen|first2=Mohua|last3=Zhou|first3=Mingfei|last4=Andrada|first4=Diego M.|last5=Frenking|first5=Gernot|title=Experimental and Theoretical Studies of the Infrared Spectra and Bonding Properties of NgBeCO<sub>3</sub> and a Comparison with NgBeO (Ng = He, Ne, Ar, Kr, Xe)|journal=The Journal of Physical Chemistry A|date=19 March 2015|volume=119|issue=11|pages=2543–2552|doi=10.1021/jp509006u|pmid=25321412|bibcode=2015JPCA..119.2543Z|url=https://figshare.com/articles/journal_contribution/2184721}}</ref>

The cyclic beryllium sulfite molecule can also coordinate an argon atom onto the beryllium atom in solid neon or argon matrix.<ref>{{cite journal |last1=Yu |first1=Wenjie |last2=Liu |first2=Xing |last3=Xu |first3=Bing |last4=Xing |first4=Xiaopeng |last5=Wang |first5=Xuefeng |title=Infrared Spectra of Novel NgBeSO2 Complexes (Ng = Ne, Ar, Kr, Xe) in Low Temperature Matrixes |journal=The Journal of Physical Chemistry A |date=21 October 2016 |volume=120 |issue=43 |pages=8590–8598 |doi=10.1021/acs.jpca.6b08799|pmid=27723974 |bibcode=2016JPCA..120.8590Y }}</ref>

==Carbonyl compounds==
[[Group 6 element]]s can form reactive penta[[carbonyl]]s that can react with argon. These were actually argon compounds discovered in 1975, and were known before the discovery of HArF, but are usually overlooked.<ref>{{cite journal|last1=Young|first1=Nigel A.|title=Main group coordination chemistry at low temperatures: A review of matrix isolated Group 12 to Group 18 complexes|journal=Coordination Chemistry Reviews|date=March 2013|volume=257|issue=5–6|pages=956–1010|doi=10.1016/j.ccr.2012.10.013}}</ref> [[Tungsten]] normally forms a [[tungsten hexacarbonyl|hexacarbonyl]], but when subject to ultraviolet radiation it breaks into a reactive pentacarbonyl. When this is condensed into a noble gas matrix the infrared and UV spectrum varies considerably depending on the noble gas used. This is because the noble gas present binds to the vacant position on the tungsten atom. Similar results also occur with [[molybdenum]] and [[chromium]].<ref name=ehl/> Argon is only very weakly bound to tungsten in ArW(CO)<sub>5</sub>.<ref name="andrews"/><ref>{{cite journal|last1=Wells|first1=J. R.|last2=Weitz|first2=Eric|title=Rare gas-metal carbonyl complexes: bonding of rare gas atoms to the Group VIB pentacarbonyls|journal=Journal of the American Chemical Society|date=April 1992|volume=114|issue=8|pages=2783–2787|doi=10.1021/ja00034a003}}</ref> The Ar−W bondlength is predicted to be 2.852&nbsp;Å.<ref name=ehl>{{cite journal|last1=Ehlers|first1=Andreas W.|last2=Frenking|first2=Gernot|last3=Baerends|first3=Evert Jan|title=Structure and Bonding of the Noble Gas−Metal Carbonyl Complexes M(CO)<sub>5</sub>−Ng (M = Cr, Mo, W and Ng = Ar, Kr, Xe)|journal=Organometallics|date=October 1997|volume=16|issue=22|pages=4896–4902|doi=10.1021/om9704962}}</ref> The same substance is produced for a brief time in [[Supercritical fluid|supercritical]] argon at 21&nbsp;°C.<ref>{{cite journal|last1=Sun|first1=Xue-Zhong|last2=George|first2=Michael W.|last3=Kazarian|first3=Sergei G.|last4=Nikiforov|first4=Sergei M.|last5=Poliakoff|first5=Martyn|title=Can Organometallic Noble Gas Compounds Be Observed in Solution at Room Temperature? A Time-Resolved Infrared (TRIR) and UV Spectroscopic Study of the Photochemistry of M(CO)<sub>5</sub> (M = Cr, Mo, and W) in Supercritical Noble Gas and CO2 Solution|journal=Journal of the American Chemical Society|date=January 1996|volume=118|issue=43|pages=10525–10532|doi=10.1021/ja960485k}}</ref> For ArCr(CO)<sub>5</sub> the band maximum is at 533&nbsp;nm (compared to 624&nbsp;nm in [[neon]], and 518&nbsp;nm in [[krypton]]). Forming [[18-electron]] complexes, the shift in spectrum due to different matrices was much smaller, only around 5&nbsp;nm. This clearly indicates the formation of a molecule using atoms from the matrix.<ref name="Grills"/>

Other carbonyls and complexed carbonyls also have reports of bonding to argon. These include Ru(CO)<sub>2</sub>(PMe<sub>3</sub>)<sub>2</sub>Ar, Ru(CO)<sub>2</sub>([[1,2-Bis(dimethylphosphino)ethane|dmpe]])<sub>2</sub>Ar, ''η''<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>Cr(CO)<sub>2</sub>Ar.<ref>{{cite journal|last1=Hope|first1=Eric G.|title=Coordination chemistry of the noble gases and noble gas fluorides|journal=Coordination Chemistry Reviews|date=March 2013|volume=257|issue=5–6|pages=902–909|doi=10.1016/j.ccr.2012.07.017}}</ref> Evidence also exists for ArHMn(CO)<sub>4</sub>, ArCH<sub>3</sub>Mn(CO)<sub>4</sub>, and ''fac''-(''η''<sup>2</sup>-dfepe)Cr(CO)<sub>3</sub>Ar.<ref name="Grills"/>

Other noble gas complexes have been studied by photolysis of carbonyls dissolved in liquid rare gas, possibly under pressure. These Kr or Xe complexes decay on the time scale of seconds, but argon does not seem to have been studied this way. The advantage of liquid noble gases is that the medium is completely transparent to [[infrared]] radiation, which is needed to study the bond vibration in the solute.<ref name="Grills"/>

Attempts have been made to study carbonyl–argon adducts in the gas phase, but the interaction appears to be too weak to observe a spectrum. In the gas form, the absorption lines are broadened into bands because of rotation that happens freely in a gas.<ref name="Grills"/> The argon adducts in liquids or gases are unstable as the molecules easily react with the other photolysis products, or [[Dimer (chemistry)|dimerize]], eliminating argon.<ref name="Grills"/>

==Coinage metal monohalides==
The argon [[coinage metal]] monohalides were the first noble gas metal halides discovered, when the metal monohalide molecules were put through an argon jet. There were first found in Vancouver in 2000.<ref>{{cite journal|last1=Grochala|first1=Wojciech|s2cid=13525010|title=Atypical compounds of gases, which have been called 'noble'|journal=Chemical Society Reviews|date=2007|volume=36|issue=10|pages=1632–1655|doi=10.1039/B702109G|pmid=17721587}}</ref>
ArMX with M = [[copper|Cu]], [[silver|Ag]] or [[gold|Au]] and X = [[fluorine|F]],<ref>{{Cite journal|last1=Wang|first1=Xuefeng|last2=Andrews|first2=Lester|last3=Willmann|first3=Knut|last4=Brosi|first4=Felix|last5=Riedel|first5=Sebastian|date=2012-10-15|title=Investigation of Gold Fluorides and Noble Gas Complexes by Matrix-Isolation Spectroscopy and Quantum-Chemical Calculations|url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201205072|journal=Angewandte Chemie International Edition|language=en|volume=51|issue=42|pages=10628–10632|doi=10.1002/anie.201205072|pmid=23001952}}</ref> [[chlorine|Cl]] or [[bromine|Br]] have been prepared. The molecules are linear. In ArAuCl the Ar−Au bond is 2.47&nbsp;Å, the stretching frequency is 198&nbsp;cm<sup>−1</sup> and the dissociation energy is 47&nbsp;kJ/mol.<ref name=evan>{{cite journal|last1=Evans|first1=Corey J.|last2=Lesarri|first2=Alberto|last3=Gerry|first3=Michael C. L.|title=Noble Gas−Metal Chemical Bonds. Microwave Spectra, Geometries, and Nuclear Quadrupole Coupling Constants of Ar−AuCl and Kr−AuCl|journal=Journal of the American Chemical Society|date=June 2000|volume=122|issue=25|pages=6100–6105|doi=10.1021/ja000874l}}</ref> ArAgBr also has been made.<ref name=evan/> ArAgF has a dissociation energy of 21&nbsp;kJ/mol.<ref name=evan/> The Ar−Ag bond-length in these molecules is 2.6&nbsp;Å.<ref name=evan/> ArAgCl is isoelectronic with {{chem|AgCl|2|−}} which is better known.<ref name=evan/> The Ar−Cu bond length in these molecules is 2.25&nbsp;Å.<ref name=evan/>

==Transition metal oxides==
In a solid argon matrix [[vanadium dioxide|VO<sub>2</sub>]] forms VO<sub>2</sub>Ar<sub>2</sub>, and VO<sub>4</sub> forms VO<sub>4</sub>·Ar with binding energy calculated to be 12.8 and 5.0&nbsp;kcal/mol (53 and 21&nbsp;kJ/mol).<ref name="ReferenceA">{{cite journal|last1=Zhao|first1=Yanying|last2=Gong|first2=Yu|last3=Chen|first3=Mohua|last4=Zhou|first4=Mingfei|title=Noble Gas−Transition-Metal Complexes: Coordination of VO<sub>2</sub> and VO<sub>4</sub> by Ar and Xe Atoms in Solid Noble Gas Matrixes|journal=The Journal of Physical Chemistry A|date=February 2006|volume=110|issue=5|pages=1845–1849|doi=10.1021/jp056476s|pmid=16451016|bibcode=2006JPCA..110.1845Z}}</ref> [[Scandium]] in the form of ScO<sup>+</sup> coordinates five argon atoms to yield {{chem|ScOAr|5|+}}.<ref>{{cite journal|last1=Zhao|first1=Yanying|last2=Wang|first2=Guanjun|last3=Chen|first3=Mohua|last4=Zhou|first4=Mingfei|title=Noble Gas−Transition Metal Complexes: Coordination of ScO<sup>+</sup> by Multiple Ar, Kr, and Xe Atoms in Noble Gas Matrixes|journal=The Journal of Physical Chemistry A|date=August 2005|volume=109|issue=30|pages=6621–6623|doi=10.1021/jp053148j|pmid=16834012|bibcode=2005JPCA..109.6621Z}}</ref> these argon atoms can be substituted by numbers of krypton or xenon atoms to yield even more mixed noble gas molecules. With [[yttrium]], YO<sup>+</sup> bonds six argon atoms, and these too can be substituted by varying numbers of krypton or xenon atoms.<ref>{{cite journal|last1=Zhao|first1=Yanying|last2=Gong|first2=Yu|last3=Chen|first3=Mohua|last4=Ding|first4=Chuanfan|last5=Zhou|first5=Mingfei|title=Coordination of ScO<sup>+</sup> and YO<sup>+</sup> by Multiple Ar, Kr, and Xe Atoms in Noble Gas Matrixes: A Matrix Isolation Infrared Spectroscopic and Theoretical Study|journal=The Journal of Physical Chemistry A|date=December 2005|volume=109|issue=51|pages=11765–11770|doi=10.1021/jp054517e|pmid=16366626|bibcode=2005JPCA..10911765Z}}</ref>

In the case of transition metal monoxides, ScO, TiO and VO do not form a molecule with one argon atom. However CrO, MnO, FeO, CoO and NiO can each coordinate one argon atom in a solid argon matrix.<ref name=zhao06>{{cite journal|last1=Zhao|first1=Yanying|last2=Gong|first2=Yu|last3=Zhou|first3=Mingfei|title=Matrix Isolation Infrared Spectroscopic and Theoretical Study of NgMO (Ng = Ar, Kr, Xe; M = Cr, Mn, Fe, Co, Ni) Complexes|journal=The Journal of Physical Chemistry A|date=September 2006|volume=110|issue=37|pages=10777–10782|doi=10.1021/jp064100o|pmid=16970371|bibcode=2006JPCA..11010777Z}}</ref> The metal monoxide molecules can be produced by laser ablation of the metal trioxide, followed by condensation on solid argon. ArCrO absorbs at 846.3&nbsp;cm<sup>−1</sup>, ArMnO at 833.1, ArFeO at 872.8, ArCoO at 846.2, Ar<sup>58</sup>NiO at 825.7 and Ar<sup>60</sup>NiO at 822.8&nbsp;cm<sup>−1</sup>. All these molecules are linear.<ref name=zhao06/>

There are also claims of argon forming coordination molecules in NbO<sub>2</sub>Ar<sub>2</sub>, NbO<sub>4</sub>Ar, TaO<sub>4</sub>Ar,<ref>{{cite journal|last1=Zhao|first1=Yanying|last2=Zheng|first2=Xuming|last3=Zhou|first3=Mingfei|title=Coordination of niobium and tantalum oxides by Ar, Xe and O<sub>2</sub>: Matrix isolation infrared spectroscopic and theoretical study of NbO<sub>2</sub>(Ng)<sub>2</sub> (Ng=Ar, Xe) and MO<sub>4</sub>(X) (M=Nb, Ta; X=Ar, Xe, O<sub>2</sub>) in solid argon|journal=Chemical Physics|date=July 2008|volume=351|issue=1–3|pages=13–18|doi=10.1016/j.chemphys.2008.03.026|bibcode=2008CP....351...13Z}}</ref> VO<sub>2</sub>Ar<sub>2</sub>, VO<sub>4</sub>Ar,<ref name="ReferenceA"/> Rh(''η''<sup>2</sup>-O<sub>2</sub>)Ar<sub>2</sub>, Rh(''η''<sup>2</sup>-O<sub>2</sub>)<sub>2</sub>Ar<sub>2</sub>, Rh(''η''<sup>2</sup>-O<sub>2</sub>)<sub>2</sub>(''η''<sup>1</sup>-OO)Ar.<ref>{{cite journal|last1=Yang|first1=Rui|last2=Gong|first2=Yu|last3=Zhou|first3=Han|last4=Zhou|first4=Mingfei|title=Matrix Isolation Infrared Spectroscopic and Theoretical Study of Noble Gas Coordinated Rhodium−Dioxygen Complexes|journal=The Journal of Physical Chemistry A|date=January 2007|volume=111|issue=1|pages=64–70|doi=10.1021/jp0662000|pmid=17201389|bibcode=2007JPCA..111...64Y}}</ref><ref>{{cite journal|last1=Brock|first1=D. S.|last2=Schrobilgen|first2=G. J.|title=Noble-Gas Chemistry|journal=Comprehensive Inorganic Chemistry II|date=9 October 2014|volume= 1: Main-Group Elements, Including Noble Gases|issue=2|pages=755–822|doi=10.1016/B978-0-08-097774-4.00128-5}}</ref><ref name=zh10>{{cite journal|last1=Zhao|first1=YanYing|last2=Zhou|first2=MingFei|title=Are matrix isolated species really "isolated"? Infrared spectroscopic and theoretical studies of noble gas–transition metal oxide complexes|journal=Science China Chemistry|date=21 February 2010|volume=53|issue=2|pages=327–336|doi=10.1007/s11426-010-0044-9|s2cid=98517909}}</ref>

[[Tungsten trioxide]], WO<sub>3</sub>, and tungsten dioxide mono-superoxide (η<sup>2</sup>-O<sub>2</sub>)WO<sub>2</sub> can both coordinate argon in an argon matrix. The argon can be replaced by xenon or molecular oxygen to make xenon coordinated compounds or superoxides. For WO<sub>3</sub>Ar the binding energy is 9.4 kcal/mol and for (η<sup>2</sup>-O<sub>2</sub>)WO<sub>2</sub> it is 8.1 kcal/mol.<ref>{{cite journal |last1=Zhou |first1=Zhaoman |last2=Zhao |first2=Yanying |title=Noble-Gas-Tungsten Peroxide Complexes in Noble Gas Matrices: Infrared Spectroscopy and Density Functional Theoretical Study |journal=The Journal of Physical Chemistry A |volume=123 |issue=2 |pages=556–564 |date=20 December 2018 |doi=10.1021/acs.jpca.8b10784|pmid=30571114 |s2cid=58603219 }}</ref>

==Other transition metal compounds==
ArNiN<sub>2</sub> binds argon with 11.52&nbsp;kcal/mol. The bending frequency of ArN<sub>2</sub> is changed from 310.7 to 358.7&nbsp;cm<sup>−1</sup> when argon attaches to the nickel atom.<ref>{{cite journal|last1=Ono|first1=Yuriko|last2=Taketsugu|first2=Tetsuya|title=Theoretical study of Ng–NiN<sub>2</sub> (Ng=Ar,Ne,He)|journal=The Journal of Chemical Physics|date=2004|volume=120|issue=13|pages=6035–40|doi=10.1063/1.1650310|pmid=15267486|bibcode=2004JChPh.120.6035O}}</ref>

==Other ions==
Some other binary ions observed that contain argon include [[Monoargobarium(II)|BaAr<sup>2+</sup>]] and {{chem|link=Diargobarium(II)|BaAr|2|2+}},<ref>{{cite journal|last1=Koyanagi|first1=Gregory K.|last2=Bohme|first2=Diethard K.|title=Strong Closed-Shell Interactions: Observed Formation of BaRg<sup>2+</sup> Molecules in the Gas Phase at Room Temperature|journal=The Journal of Physical Chemistry Letters|date=7 January 2010|volume=1|issue=1|pages=41–44|doi=10.1021/jz900009q}}</ref> [[Argovanadium(I)|VAr<sup>+</sup>]], [[Argochromium(I)|CrAr<sup>+</sup>]], [[Argoiron(I)|FeAr<sup>+</sup>]], [[Argocopper(I)|CoAr<sup>+</sup>]], and [[Argonickel(I)|NiAr<sup>+</sup>]].<ref name="Grills"/>

Gold and silver cluster ions can bind argon. Known ions are {{chem|link=Monoargotrigold(I)|Au|3|Ar|+}}, {{chem|link=Diargotrigold(I)|Au|3|Ar|2|+}}, {{chem|link=Triargotrigold(I)|Au|3|Ar|3|+}}, {{chem|link=Gold(II) Triargoargentate|Au|2|AgAr|3|+}} and {{chem|link=Triargodiargentogold(I)|AuAg|2|Ar|3|+}}. These have a triangular shaped metallic core with argon bound at the vertexes.<ref name=gold/>

ArF<sup>+</sup> is also known<ref name="Grills">{{Cite book|last1=Grills|first1=D. C.|last2=George|first2=M. W.|title=Transition metal–noble gas complexes|date=2001|volume=52|pages=113–150|doi=10.1016/S0898-8838(05)52002-6|series=Advances in Inorganic Chemistry|isbn=9780120236527}}</ref> to be formed in the reaction
:{{chem|F|2|+}} + Ar → ArF<sup>+</sup> + F
and also
:Ar<sup>+</sup> + F<sub>2</sub> → ArF<sup>+</sup> + F.
and also
:SF{{su|b=4|p=2+}} + Ar → ArF<sup>+</sup> + SF{{su|b=3|p=+}}.<ref>{{cite journal|last1=Lockyear|first1=Jessica F.|last2=Parkes|first2=Michael A.|last3=Price|first3=Stephen D.|title=Fast and Efficient Fluorination of Small Molecules by SF42+|journal=Angewandte Chemie|date=7 February 2011|volume=123|issue=6|pages=1358–1360|doi=10.1002/ange.201006486|bibcode=2011AngCh.123.1358L}}</ref>
The ions can be produced by ultraviolet light at 79.1&nbsp;nm or less.<ref>{{cite journal|last1=Berkowitz|first1=J.|last2=Chupka|first2=W. A.|title=Diatomic ions of noble gas fluorides|journal=Chemical Physics Letters|date=November 1970|volume=7|issue=4|pages=447–450|doi=10.1016/0009-2614(70)80331-1|bibcode=1970CPL.....7..447B}}</ref> The ionisation energy of fluorine is higher than that of argon, so breakup occurs thus:
:ArF<sup>+</sup> → Ar<sup>+</sup> + F.<ref name=frenking>{{cite journal|last1=Frenking|first1=Gernot|last2=Koch|first2=Wolfram|last3=Deakyne|first3=Carol A.|last4=Liebman|first4=Joel F.|last5=Bartlett|first5=Neil|title=The ArF<sup>+</sup> cation. Is it stable enough to be isolated in a salt?|journal=Journal of the American Chemical Society|date=January 1989|volume=111|issue=1|pages=31–33|doi=10.1021/ja00183a005}}</ref>
The millimeter wave spectrum of ArF<sup>+</sup> between 119.0232 and 505.3155&nbsp;GHz has been measured to calculate molecular constants ''B''<sub>0</sub>&nbsp;=&nbsp;{{val|14.8788204|u=GHz}}, ''D''<sub>0</sub>&nbsp;=&nbsp;28.718&nbsp;kHz.<ref>{{cite journal|last1=Bogey|first1=M.|last2=Cordonnier|first2=M.|last3=Demuynck|first3=C.|last4=Destombes|first4=J. L.|year=1992|title=The millimeter- and submillimeter-wave spectrum of ArF<sup>+</sup>|journal=Journal of Molecular Spectroscopy|volume=155|issue=1|pages= 217–219|doi=10.1016/0022-2852(92)90562-3|bibcode=1992JMoSp.155..217B}}</ref> There is a possibility that a solid salt of ArF<sup>+</sup> could be prepared with [[Hexafluoroantimonate|{{chem|SbF|6|−}}]] or [[hexafluoroaurate|{{chem|AuF|6|−}}]] anions.<ref name=frenking/><ref>{{Cite book|last1=Selig|first1=Henry|last2=Holloway|first2=John H.|title=Cationic and anionic complexes of the noble gases|date=27 May 2005|volume=124|pages=33–90|doi=10.1007/3-540-13534-0_2|series=Topics in Current Chemistry|isbn=978-3-540-13534-0|s2cid=91636049 }}</ref>

Excited or ionized argon atoms can react with molecular iodine gas to yield ArI<sup>+</sup><ref name=henglein>{{cite journal|last1=Henglein|first1=A.|last2=Muccini|first2=G. A.|title=Neue Ion–Molekül-Reaktionen und ihre Bedeutung für die Strahlenchemie|trans-title=New ion–molecule reactions and their importance for radiation chemistry|journal=Berichte der Bunsengesellschaft für Physikalische Chemie|date=October 1960|volume=64|issue=8–9|issn=0005-9021|language=de|page=1015|doi=10.1002/bbpc.19600640807|s2cid=98595129 |url=https://onlinelibrary.wiley.com/doi/10.1002/bbpc.19600640807}}</ref>
Argon plasma is used as an ionisation source and carrier gas in [[inductively coupled plasma mass spectrometry]]. This plasma reacts with samples to produce monatomic ions, but also forms argon oxide (ArO<sup>+</sup>), and argon nitride (ArN<sup>+</sup>) cations, which can cause isobaric interference with detection and measurement of [[iron-56]] (<sup>56</sup>Fe) and [[iron-54]] (<sup>54</sup>Fe), respectively, in mass spectrometry.<ref name=alcock>{{cite journal|last1=Alcock|first1=Nancy|title=Flame, Flameless, and Plasma Spectroscopy|journal=Analytical Chemistry|date=1 June 1993|volume=65|issue=12|pages=463–469|doi=10.1021/ac00060a618|issn=0003-2700}}</ref> Platinum present in stainless steel can form platinum argide (PtAr<sup>+</sup>) which interferes with the detection of [[uranium-234]] which can be used as a tracer in aquifers.<ref>{{cite journal|last1=Mas|first1=José Luis|last2=Ma|first2=Renli|last3=McLeod|first3=Cameron|last4=González-Labajo|first4=Jesús|last5=Cox|first5=Alan|last6=Watson|first6=Paul|title=Determination of 234U/238U isotope ratios in environmental waters by quadrupole ICP-MS after U stripping from alpha-spectrometry counting sources|journal=Analytical and Bioanalytical Chemistry|date=26 July 2006|volume=386|issue=1|pages=152–160|doi=10.1007/s00216-006-0601-4|pmid=16868729|s2cid=46457224}}</ref> Argon chloride cations can interfere with the detection of [[arsenic]] as Ar<sup>35</sup>Cl<sup>+</sup> has a mass-to-charge ratio almost identical to that of [[isotopes of arsenic|arsenic's one stable isotope]], <sup>75</sup>As.<ref>{{cite journal|last1=Sheppard|first1=Brenda S.|last2=Shen|first2=Wei-Lung|last3=Caruso|first3=Joseph A.|last4=Heitkemper|first4=Douglas T.|last5=Fricke|first5=Fred L.|title=Elimination of the argon chloride interference on arsenic speciation in inductively coupled plasma mass spectrometry using ion chromatography|journal=Journal of Analytical Atomic Spectrometry|date=1990|volume=5|issue=6|pages=431|doi=10.1039/JA9900500431}}</ref> In these circumstances ArO<sup>+</sup> may be removed by reaction with NH<sub>3</sub>.<ref>{{cite journal|last1=MacKay|first1=M.|last2=Rusho|first2=W.|last3=Jackson|first3=D.|last4=McMillin|first4=G.|last5=Winther|first5=B.|title=Physical and Chemical Stability of Iron Sucrose in Parenteral Nutrition|journal=Nutrition in Clinical Practice|date=2 December 2009|volume=24|issue=6|pages=733–737|doi=10.1177/0884533609351528|pmid=19955552}}</ref> Alternatively [[Electro-Thermal Vaporisation|electrothermal vaporization]] or using helium gas can avoid these interference problems.<ref name=alcock/> Argon can also form an anion with chlorine, ArCl<sup>−</sup>,<ref>{{cite journal|last1=Lenzer|first1=Thomas|last2=Yourshaw|first2=Ivan|last3=Furlanetto|first3=Michael R.|last4=Reiser|first4=Georg|last5=Neumark|first5=Daniel M.|s2cid=29240758|title=Zero electron kinetic energy spectroscopy of the ArCl<sup>−</sup> anion|journal=The Journal of Chemical Physics|date=1999|volume=110|issue=19|pages=9578|doi=10.1063/1.478923|bibcode=1999JChPh.110.9578L}}</ref> though this is not a problem for mass spectrometry applications as only cations are detected.

The argon borynium ion, BAr<sup>+</sup> is produced when BBr<sup>+</sup> at energies between 9 and 11&nbsp;eV reacts with argon atoms. 90% of the positive charge is on the argon atom.<ref name=Koskinen>{{cite journal|last1=Koskinen|first1=Jere T.|title=Novel Rare Gas Ions BXe<sup>+</sup>, BKr<sup>+</sup>, and BAr<sup>+</sup> Formed in a Halogen/Rare Gas Exchange Reaction|journal=The Journal of Physical Chemistry A|date=December 1999|volume=103|issue=48|pages=9565–9568|doi=10.1021/jp993091z|bibcode=1999JPCA..103.9565K}}</ref>

ArC<sup>+</sup> ions can be formed when argon ions impact carbon monoxide with energies between 21 and 60&nbsp;eV. However more C<sup>+</sup> ions are formed, and when the energy is on the high side, O<sup>+</sup> is higher.<ref>{{cite journal|last1=Flesch|first1=G. D.|last2=Ng|first2=C. Y.|title=Observation of the formation of C<sup>+</sup>, O<sup>+</sup>, and ArC<sup>+</sup> in the collisions of Ar<sup>+</sup>(<sup>2</sup>P<sub>3/2,1/2</sub>) with CO|journal=The Journal of Chemical Physics|date=1988|volume=89|issue=5|pages=3381|doi=10.1063/1.455708|bibcode=1988JChPh..89.3381F}}</ref>

ArN<sup>+</sup> can form when argon ions impact [[dinitrogen]] with energies between 8.2 and 41.2&nbsp;eV and peaking around 35&nbsp;eV. However far more {{chem|N|2|+}} and N<sup>+</sup> are produced.<ref>{{cite journal|last1=Flesch|first1=G. D.|last2=Ng|first2=C. Y.|title=Observation of the formation of N<sup>+</sup> and ArN<sup>+</sup> in the collisions of Ar<sup>+</sup>(<sup>2</sup>P<sub>3/2,1/2</sub>) with N<sub>2</sub>|journal=The Journal of Chemical Physics|date=1990|volume=92|issue=5|pages=2876|doi=10.1063/1.457934|bibcode=1990JChPh..92.2876F}}</ref>

ArXe<sup>+</sup> is held together with a strength of 1445&nbsp;cm<sup>−1</sup> when it is in the X electronic state, but 1013&nbsp;cm<sup>−1</sup> when it is in the B excited state.<ref name=n2o/>

Metal–argon cations are called "argides". The argide ions produced during [[mass spectroscopy]] have higher intensity when the binding energy of the ion is higher. Transition elements have higher binding and ion flux intensity compared to main group elements. Argides can be formed in the plasma by excited argon atoms reacting with another element atom, or by an argon atom binding with another ion:
:Ar<sup>+</sup> + M → ArM<sup>+</sup> + e<sup>−</sup>; M<sup>+</sup> + Ar → ArM<sup>+</sup>.<ref name=becker>{{cite journal|last1=Becker|first1=J. Sabine|last2=Seifert|first2=Gotthard|last3=Saprykin|first3=Anatoli I.|last4=Dietze|first4=Hans-Joachim|title=Mass spectrometric and theoretical investigations into the formation of argon molecular ions in plasma mass spectrometry|journal=Journal of Analytical Atomic Spectrometry|date=1996|volume=11|issue=9|pages=643|doi=10.1039/JA9961100643}}</ref>

Doubly charged cations, called [[superelectrophile]]s, are capable of reacting with argon. Ions produced include ArCF{{su|b=2|p=2+}} ArCH{{su|b=2|p=+}}, ArBF{{su|b=2|p=+}} and ArBF{{su|p=2+}} containing bonds between argon and carbon or boron.<ref name="lev">{{cite journal|last1=Levee|first1=Lauren|last2=Calogero|first2=Catherine|last3=Barbieri|first3=Edward|last4=Byrne|first4=Steven|last5=Donahue|first5=Courtney|last6=Eisenberg|first6=Michael|last7=Hattenbach|first7=Sean|last8=Le|first8=Julie|last9=Capitani|first9=Joseph F.|last10=Roithová|first10=Jana|last11=Schröder|first11=Detlef|title=Formation of argon–boron bonds in the reactions of BFn+/2+ cations with neutral argon|journal=International Journal of Mass Spectrometry|date=June 2012|volume=323–324|pages=2–7|doi=10.1016/j.ijms.2012.05.006|bibcode=2012IJMSp.323....2L}}</ref>

Doubly ionised acetylene HCCH<sup>2+</sup> reacts inefficiently with argon to yield HCCAr<sup>2+</sup>.  This product competes with the formation of Ar<sup>+</sup> and argonium.<ref>{{cite journal|last1=Ascenzi|first1=Daniela|last2=Tosi|first2=Paolo|last3=Roithová|first3=Jana|last4=Ricketts|first4=Claire L.|last5=Schröder|first5=Detlef|last6=Lockyear|first6=Jessica F.|last7=Parkes|first7=Michael A.|last8=Price|first8=Stephen D.|title=Generation of the organo-rare gas dications HCCRg2+ (Rg = Ar and Kr) in the reaction of acetylene dications with rare gases|journal=Physical Chemistry Chemical Physics|date=2008|volume=10|issue=47|pages=7121–8|doi=10.1039/B810398D|pmid=19039346|bibcode=2008PCCP...10.7121A}}</ref>

The SiF{{su|p=2+|b=3}} ion reacts with argon to yield ArSiF{{su|p=2+|b=2}}.<ref>{{cite journal|last1=Roithová|first1=Jana|last2=Schröder|first2=Detlef|title=Siliciumverbindungen von Neon und Argon|journal=Angewandte Chemie|date=2 November 2009|volume=121|issue=46|pages=8946–8948|language=de|doi=10.1002/ange.200903706|bibcode=2009AngCh.121.8946R}}</ref>

:{|class="wikitable"
!Ion
!Bond length<br/>(Å)
!Dissociation energy<br>(kJ/mol)<ref name="Grills"/>
!Excited state<br/>bond length (Å)
!Excited state<br/>dissociation energy
|-
|ArH<sup>+</sup>
|
|3.4&nbsp;eV
|-
|LiAr<sup>+</sup><ref name=becker/>
|2.343
|0.30&nbsp;eV
|-
|BeAr<sup>+</sup><ref name=becker/>
|
|4100&nbsp;cm<sup>−1</sup><ref name="luder">{{cite journal|last1=Lüder|first1=Ch.|last2=Velegrakis|first2=M.|title=Photofragmentation spectrum of the Sr+Ar complex|journal=The Journal of Chemical Physics|date=1996|volume=105|issue=6|pages=2167|doi=10.1063/1.472090|bibcode=1996JChPh.105.2167L}}</ref>
|-
|BAr<sup>+</sup><ref name=Koskinen/>
|2.590
|210<!--50.1 kcal/mol-->
|-
|ArC<sup>+</sup><ref>{{cite journal|last1=Hillier|first1=I. H.|last2=Guest|first2=M. F.|last3=Ding|first3=A.|last4=Karlau|first4=J.|last5=Weise|first5=J.|title=The potential energy curves of ArC<sup>+</sup>|journal=The Journal of Chemical Physics|date=1979|volume=70|issue=2|pages=864|doi=10.1063/1.437518|bibcode=1979JChPh..70..864H}}</ref>
|
|
|-
|ArN<sup>+</sup><ref name=becker/>
|3.5
|2.16&nbsp;eV<ref>{{cite journal|last1=Broström|first1=L.|last2=Larsson|first2=M.|last3=Mannervik|first3=S.|last4=Sonnek|first4=D.|title=The visible photoabsorption spectrum and potential curves of ArN<sup>+</sup>|journal=The Journal of Chemical Physics|date=1991|volume=94|issue=4|pages=2734|doi=10.1063/1.459850|bibcode=1991JChPh..94.2734B}}</ref>
|-
|ArO<sup>+</sup><ref name=becker/>
|
|
|-
|ArF<sup>+</sup><ref name=frenking/>
|1.637
|194
|-
|NaAr<sup>+</sup><ref name=becker/>
|
|19.3<!-- 0.2eV -->
|-
|[[Magnesium argide|MgAr<sup>+</sup>]]<ref name=becker/>
|2.88<ref name=but/>
|1200&nbsp;cm<sup>−1</sup><ref name="luder"/>
|-
|AlAr<sup>+</sup><ref name=becker/>
|
|982&nbsp;cm<sup>−1</sup><ref>{{cite journal|last1=Pilgrim|first1=J. S.|last2=Yeh|first2=C. S.|last3=Berry|first3=K. R.|last4=Duncan|first4=M. A.|title=Photodissociation spectroscopy of Mg+–rare gas complexes|journal=The Journal of Chemical Physics|date=1994|volume=100|issue=11|pages=7945|doi=10.1063/1.466840|bibcode=1994JChPh.100.7945P}}</ref>
|-
|SiAr<sup>+</sup><ref name=becker/>
|
|
|-
|ArP<sup>+</sup><ref name=becker/>
|
|
|-
|ArS<sup>+</sup><ref name=becker/>
|
|
|-
|ArCl<sup>+</sup><ref name=becker/>
|
|
|-
|{{chem|Ar|2|+}}<ref name=becker/>
|
|
|-
|CaAr<sup>+</sup>
|
|700&nbsp;cm<sup>−1</sup><ref name="luder"/>
|-
|ScAr<sup>+</sup><ref name=becker/>
|
|
|-
|TiAr<sup>+</sup>
|
|0.31eV<ref name="wit">{{cite journal|last1=Witte|first1=Travis M.|last2=Houk|first2=R.S.|title=Metal argide (MAr+) ions are lost during ion extraction in laser ablation-inductively coupled plasma-mass spectrometry|journal=Spectrochimica Acta Part B: Atomic Spectroscopy|date=March 2012|volume=69|pages=25–31|doi=10.1016/j.sab.2012.02.008|bibcode=2012AcSpe..69...25W|url=https://zenodo.org/record/1259345}}</ref>
|-
|VAr<sup>+</sup>
|2.65<ref name="malt">{{cite journal|last1=Maltsev|first1=M A|last2=Kulikov|first2=A N|last3=Morozov|first3=I V|title=Thermodynamic properties of vanadium and cobalt argide ions, VAr+ and CoAr+|journal=Journal of Physics: Conference Series|date=November 2016|volume=774|issue=1|pages=012023|doi=10.1088/1742-6596/774/1/012023|bibcode=2016JPhCS.774a2023M|doi-access=free}}</ref>
|37,D<sub>0</sub>=2974&nbsp;cm<sup>−1</sup><ref name=but/>
|-
|CrAr<sup>+</sup>
|
|28,D<sub>0</sub>=2340<ref name=but/>
|-
|MnAr<sup>+</sup><ref name=becker/>
|
|0.149&nbsp;eV<ref name="wit"/>
|-
|FeAr<sup>+</sup>
|
|0.11&nbsp;eV<ref name="wit"/>
|-
|CoAr<sup>+</sup><ref name="malt"/>
|2.385<ref name=bel>{{cite journal|last1=Asher|first1=R.L.|last2=Bellert|first2=D.|last3=Buthelezi|first3=T.|last4=Brucat|first4=P.J.|title=The ground state of CoAr+|journal=Chemical Physics Letters|date=September 1994|volume=227|issue=3|pages=277–282|doi=10.1016/0009-2614(94)00828-0|bibcode=1994CPL...227..277A}}</ref>
|49,D<sub>0</sub>=4111&nbsp;cm<sup>−1</sup><ref name=bel/>
|-
|NiAr<sup>+</sup>
|
|53,D<sub>0</sub>=4572<ref name=but/>
|-
|CuAr<sup>+</sup><ref name=becker/>
|
|0.53&nbsp;eV<ref name="wit"/>
|-
|ZnAr<sup>+</sup>
|2.72<ref name="ashe">{{cite journal|last1=Asher|first1=R.L.|last2=Bellert|first2=D.|last3=Buthelezi|first3=T.|last4=Lessen|first4=Dan|last5=Brucat|first5=P.J.|title=The bond length of ZrAr+|journal=Chemical Physics Letters|date=March 1995|volume=234|issue=1–3|pages=119–122|doi=10.1016/0009-2614(95)00006-P|bibcode=1995CPL...234..119A}}</ref>
|0.25&nbsp;eV,<ref name="wit"/> D<sub>0</sub>=2706&nbsp;cm<sup>−1</sup><ref name="ashe"/>
|-
|GaAr<sup>+</sup><ref name=becker/>
|
|
|-
|AsAr<sup>+</sup><ref name=becker/>
|
|
|-
|RbAr<sup>+</sup><ref name=freem>{{cite journal|last1=Freeman|first1=J. H.|last2=McIlroy|first2=R. W.|title=Preparation of Ion-molecules of the Inert Gases in an Electromagnetic Isotope Separator|journal=Nature|date=4 January 1964|volume=201|issue=4914|pages=69–70|doi=10.1038/201069a0|bibcode=1964Natur.201...69F|s2cid=4246089}}</ref>
|
|
|-
|SrAr<sup>+</sup>
|
|800<ref name="luder"/>
|-
|ZrAr<sup>+</sup>
|2.72
|D<sub>0</sub> = 2706&nbsp;cm<sup>−1</sup><ref name="ashe"/>
|3.050
|1179&nbsp;cm<sup>−1</sup>
|-
|NbAr<sup>+</sup>
|2.677<ref name=but/>
|37,D<sub>0</sub>=3106&nbsp;cm<sup>−1</sup><ref name=but>{{cite journal|last1=Buthelezi|first1=T.|last2=Bellert|first2=D.|last3=Hayes|first3=T.|last4=Brucat|first4=P.J.|title=The adiabatic binding energy of NbAr+|journal=Chemical Physics Letters|date=November 1996|volume=262|issue=3–4|pages=303–307|doi=10.1016/0009-2614(96)01095-0|bibcode=1996CPL...262..303B}}</ref>
|-
|AgAr<sup>+</sup><ref name=becker/>
|
|
|-
|InAr<sup>+</sup><ref name="luder2">{{cite journal|last1=Lüder|first1=Christian|last2=Prekas|first2=Dimitris|last3=Velegrakis|first3=Michalis|title=Ion-Size Effects in the Growth Sequences of Metal-Ion-Doped Noble Gas Clusters|journal=Laser Chemistry|date=1997|volume=17|issue=2|pages=109–122|doi=10.1155/1997/49504|doi-access=free}}</ref>
|
|
|-
|ArI<sup>+</sup><ref name=henglein/>
|
|
|-
|BaAr<sup>+</sup>
|
|600&nbsp;cm<sup>−1</sup><ref name="luder"/>
|}

===Polyatomic cations===

Metal ions can also form with more than one argon atom, in a kind of argon metal cluster. Different sized metal ions at the centre of a cluster can fit different geometries of argons atoms around the ion.<ref name="luder2"/>  Argides with multiple argon atoms have been detected in mass spectrometry. These can have variable numbers of argon attached, but there are magic numbers, where the complex more commonly has a particular number, either four or six argon atoms.<ref name=beye/> These can be studied by [[time of flight mass spectrometer]] analysis and by the [[photodissociation spectrum]]. Other study methods include [[Coulomb explosion]] analysis.<ref>{{cite journal|last1=Lezius|first1=M.|last2=Dobosz|first2=S.|last3=Normand|first3=D.|last4=Schmidt|first4=M.|title=Explosion Dynamics of Rare Gas Clusters in Strong Laser Fields|journal=Physical Review Letters|date=12 January 1998|volume=80|issue=2|pages=261–264|doi=10.1103/PhysRevLett.80.261|bibcode=1998PhRvL..80..261L}}</ref> Argon-tagging is a technique whereby argon atoms are weakly bound to a molecule under study.  It results in a much lower temperature of the tagged molecules, with sharper infra-red absorption lines.  The argon-tagged molecules can be disrupted by photons of a particular wavelength.<ref>{{cite book|last1=Falconer|first1=Travis M.|title=Ion Capture in Helium Droplets: Formation of Cold Ion-neutral Clusters|date=2008|isbn=9780549671015|page=7|url=https://books.google.com/books?id=LYYF8UaW6_MC&pg=PA7|language=en}}</ref>

Lithium ions add argon atoms to form clusters with more than a hundred argon atoms. The clusters Li<sup>+</sup>Ar<sub>4</sub>, and Li<sup>+</sup>Ar<sub>4</sub> are particularly stable and common. Calculations show that the small clusters are all quite symmetrical. Li<sup>+</sup>Ar<sub>2</sub> is linear, Li<sup>+</sup>Ar<sub>3</sub> is flat and triangular shaped with D<sub>3h</sub> symmetry, Li<sup>+</sup>Ar<sub>4</sub> is tetrahedral, Li<sup>+</sup>Ar<sub>5</sub> could be a [[Square pyramidal molecular geometry|square pyramid]] or [[Trigonal bipyramidal molecular geometry|trigonal bipyramid]] shape. Li<sup>+</sup>Ar<sub>6</sub> is an [[Octahedral molecular geometry|octahedron]] shape with Li at the centre. Li<sup>+</sup>Ar<sub>7</sub> or slightly larger clusters have a core octahedron of argon atoms with one or more triangular faces capped by other argon atoms.  The bonding is much weaker, which explains their greater scarcity.<ref>{{cite journal|last1=Froudakis|first1=George E|last2=Farantos|first2=Stavros C|last3=Velegrakis|first3=Michalis|title=Mass spectra and theoretical modeling of Li+Nen, Li+Arn and Li+Krn clusters|journal=Chemical Physics|date=August 2000|volume=258|issue=1|pages=13–20|doi=10.1016/S0301-0104(00)00175-0|bibcode=2000CP....258...13F}}</ref>

Sodium forms clusters with argon atoms with peaks at numbers of 8, 10, 16, 20, 23, 25 and 29, and also at the icosahedral numbers of 47, 50, 57, 60, 63, 77, 80, 116 and 147 argon atoms. This includes the [[Square antiprismatic molecular geometry|square antiprism]] (8) and the capped square antiprism (10 atoms).<ref name="luder2"/>
In Ti<sup>+</sup>Ar<sub>1−n</sub> the argon atoms induce a mixing of the ground electronic state of 3d<sup>2</sup>4s<sup>1</sup> with 3d<sup>3</sup>4s<sup>0</sup>.  When a plasma of titanium in expanding argon gas is made via a laser, clusters from Ti<sup>+</sup>Ar up to Ti<sup>+</sup>Ar<sub>50</sub> are formed.  But Ti<sup>+</sup>Ar<sub>6</sub> is much more common than all the others.  In this the six argon atoms are arranged in an octahedron shape around the central titanium ion. For Ti<sup>+</sup>Ar<sub>2</sub> DFT calculations predict it is linear, Ti<sup>+</sup>Ar<sub>3</sub> is not even flat, and has one short and two longer Ti-Ar bonds. Ti<sup>+</sup>Ar<sub>4</sub> is a distorted tetrahedron, with one longer Ti-Ar bond. Ti<sup>+</sup>Ar<sub>5</sub> is an asymmetrical [[Trigonal bipyramidal molecular geometry|trigonal bipyramid]] shape with one bond shorter. For clusters with seven or more argon atoms, the structure contains a Ti<sup>+</sup>Ar<sub>6</sub> octahedton with triangular faces caped by more argon atoms.<ref>{{cite journal|last1=Velegrakis|first1=Michalis|last2=Froudakis|first2=George E|last3=Farantos|first3=Stavros C|title=Coordination of Ti cation embedded in argon clusters|journal=Chemical Physics Letters|date=March 1999|volume=302|issue=5–6|pages=595–601|doi=10.1016/S0009-2614(99)00162-1|bibcode=1999CPL...302..595V}}</ref>

Cu<sup>+</sup>Ar<sub>2</sub> is predicted to be linear. Cu<sup>+</sup>Ar<sub>3</sub> is predicted to be planar T-shaped with an Ar-Cu-Ar angle of 93°. Cu<sup>+</sup>Ar<sub>4</sub> is predicted to be rhombic planar (not square or tetrahedral). For alkali and alkaline earth metals the M<sup>+</sup>Ar<sub>4</sub> cluster is tetrahedral. Cu<sup>+</sup>Ar<sub>5</sub> is predicted to have a rhombic pyramid shape. Cu<sup>+</sup>Ar<sub>6</sub> has a flattened octahedral shape. Cu<sup>+</sup>Ar<sub>7</sub> is much less stable, and the seventh argon atom is outside an inner shell of six argon atoms.  This is called capped octahedral. A complete second shell of argon atoms yields Cu<sup>+</sup>Ar<sub>34</sub>. Above this number a structural change takes place with an icosahedral arrangement with Cu<sup>+</sup>Ar<sub>55</sub> and Cu<sup>+</sup>Ar<sub>146</sub> having more stability.<ref name="Frou">{{cite journal|last1=Froudakis|first1=George E.|last2=Muhlhauser|first2=Max|last3=Farantos|first3=Stavros C.|last4=Sfounis|first4=Antonis|last5=Velegrakis|first5=Michalis|title=Mass spectra and structures of Cu+Rgn clusters (Rg=Ne, Ar)|journal=Chemical Physics|date=June 2002|volume=280|issue=1–2|pages=43–51|doi=10.1016/S0301-0104(02)00512-8|bibcode=2002CP....280...43F}}</ref>

With a strontium ion Sr<sup>+</sup> from two to eight argon atoms can form clusters. Sr<sup>+</sup>Ar<sub>2</sub> has a triangle shape with ''C''<sub>2''v''</sub> symmetry. Sr<sup>+</sup>Ar<sub>3</sub> has a trigonal pyramid shape with ''C''<sub>3''v''</sub> symmetry. Sr<sup>+</sup>Ar<sub>4</sub> has two trigonal pyramids sharing a face and strontium at the common apex. It has a ''C''<sub>2''v''</sub> symmetry. Sr<sup>+</sup>Ar<sub>6</sub> has a pentagonal pyramid of argon atoms with the strontium atom ''below'' the base.<ref>{{cite journal|last1=Fanourgakis|first1=G. S.|last2=Farantos|first2=S. C.|last3=Lüder|first3=Ch.|last4=Velegrakis|first4=M.|last5=Xantheas|first5=S. S.|s2cid=10006288|title=Photofragmentation spectra and structures of Sr[sup +]Ar[sub n], n=2–8 clusters: Experiment and theory|journal=The Journal of Chemical Physics|date=1998|volume=109|issue=1|pages=108|doi=10.1063/1.476527|bibcode=1998JChPh.109..108F}}</ref>

[[Niobium tetraargide]], Nb<sup>+</sup>Ar<sub>4</sub> probably has the argon atoms arranged in a square around the niobium. Similarly for vanadium tetraargide, V<sup>+</sup>Ar<sub>4</sub>. The hexaargides, Co<sup>+</sup>Ar<sub>6</sub> and Rh<sup>+</sup>Ar<sub>6</sub> likely have octahedral argon arrangement.<ref name=beye>{{cite journal|last1=Beyer|first1=Martin|last2=Berg|first2=Christian|last3=Albert|first3=Gerhard|last4=Achatz|first4=Uwe|last5=Bondybey|first5=Vladimir E|title=Coordinative saturation of cationic niobium– and rhodium–argon complexes|journal=Chemical Physics Letters|date=December 1997|volume=280|issue=5–6|pages=459–463|doi=10.1016/S0009-2614(97)01203-7|bibcode=1997CPL...280..459B}}</ref>
Indium monocation forms clusters with multiple argon, with magic numbers at 12, 18, 22, 25, 28, 45 and 54, and 70 argon atoms, which are numbers for icosahedral shapes.<ref name="luder2"/>

By zapping copper metal with a UV laser in an argon-carbon monoxide mixture, argon tagged copper carbonyl cations are formed. These ions can be studied by observing which wavelengths of infrared radiation cause the molecules to break up. These molecular ions include CuCO<sup>+</sup>Ar, Cu(CO)<sub>2</sub><sup>+</sup>Ar, Cu(CO)<sub>3</sub><sup>+</sup>Ar, Cu(CO)<sub>4</sub><sup>+</sup>Ar which are respectively disrupted to lose argon, by infrared wavenumbers 2216, 2221, 2205 and 2194&nbsp;cm<sup>−1</sup> respectively. The argon binding energy is respectively 16.3, 1.01, 0.97 and 0.23 kcal/mol. The infrared absorption peak for Cu(CO)<sub>3</sub><sup>+</sup>Ar is 2205&nbsp;cm<sup>−1</sup> compared to 2199&nbsp;cm<sup>−1</sup> for Cu(CO)<sub>3</sub><sup>+</sup>. For Cu(CO)<sub>4</sub><sup>+</sup>Ar the peak is at 2198&nbsp;cm<sup>−1</sup> compared to 2193 for Cu(CO)<sub>4</sub><sup>+</sup>. For Cu(CO)<sub>2</sub><sup>+</sup>Ar the peak is at 2221&nbsp;cm<sup>−1</sup> compared to 2218.3 for argon free, and for CuCO<sup>+</sup>Ar the peak is at 2216&nbsp;cm<sup>−1</sup> considerably different to 2240.6&nbsp;cm<sup>−1</sup> for CuCO<sup>+</sup>. Computationally predicted shapes for these molecular ions are linear for CuCO<sup>+</sup>Ar, slightly bent T-shaped for Cu(CO)<sub>2</sub><sup>+</sup>Ar and a trigonal pyramid with argon at the top and a flat star like copper tricarbonyl forming the base.<ref>{{cite journal|last1=Brathwaite|first1=A. D.|last2=Reed|first2=Z. D.|last3=Duncan|first3=M. A.|title=Infrared Photodissociation Spectroscopy of Copper Carbonyl Cations|journal=The Journal of Physical Chemistry A|date=29 September 2011|volume=115|issue=38|pages=10461–10469|doi=10.1021/jp206102z|pmid=21861528|bibcode=2011JPCA..11510461B}}</ref>

Ions studied by argon tagging include the hydrated proton H<sup>+</sup>(H<sub>2</sub>O)<sub>n</sub>Ar with n=2 to 5,<ref>{{cite journal|last1=Douberly|first1=G. E.|last2=Walters|first2=R. S.|last3=Cui|first3=J.|last4=Jordan|first4=K. D.|last5=Duncan|first5=M. A.|title=Infrared Spectroscopy of Small Protonated Water Clusters, H+(H2O)n n= 2–5): Isomers, Argon Tagging, and Deuteration|journal=The Journal of Physical Chemistry A|date=8 April 2010|volume=114|issue=13|pages=4570–4579|doi=10.1021/jp100778s|pmid=20232806|bibcode=2010JPCA..114.4570D}}</ref> hydrated [[18-crown-6 ether]] alkali metal ions,<ref>{{cite journal|last1=Rodriguez|first1=Jason D.|last2=Lisy|first2=James M.|title=Probing Ionophore Selectivity in Argon-Tagged Hydrated Alkali Metal Ion–Crown Ether Systems|journal=Journal of the American Chemical Society|date=27 July 2011|volume=133|issue=29|pages=11136–11146|doi=10.1021/ja107383c|pmid=21675737}}</ref> hydrated alkali metal ions,<ref>{{cite journal|last1=Rodriguez|first1=Oscar|last2=Lisy|first2=James M.|title=Revisiting Li+ (H2O)3-4 Ar Clusters: Evidence of High-Energy Conformers from Infrared Spectra|journal=The Journal of Physical Chemistry Letters|date=16 June 2011|volume=2|issue=12|pages=1444–1448|doi=10.1021/jz200530v}}</ref> transition metal acetylene complexes,<ref>{{cite journal|last1=Walters|first1=Richard S.|last2=Schleyer|first2=Paul v. R.|last3=Corminboeuf|first3=Clemence|last4=Duncan|first4=Michael A.|title=Structural Trends in Transition Metal Cation−Acetylene Complexes Revealed through the C−H Stretching Fundamentals|journal=Journal of the American Chemical Society|date=February 2005|volume=127|issue=4|pages=1100–1101|doi=10.1021/ja043766y|pmid=15669839}}</ref> protonated ethylene,<ref>{{cite journal|last1=Ricks|first1=Allen M.|last2=Douberly|first2=Gary E.|last3=Schleyer|first3=Paul v.R.|last4=Duncan|first4=Michael A.|title=Infrared spectroscopy of protonated ethylene: The nature of proton binding in the non-classical structure|journal=Chemical Physics Letters|date=September 2009|volume=480|issue=1–3|pages=17–20|doi=10.1016/j.cplett.2009.08.063|bibcode=2009CPL...480...17R}}</ref> and IrO<sub>4</sub><sup>+</sup>.<ref>{{cite journal|last1=Wang|first1=Guanjun|last2=Zhou|first2=Mingfei|last3=Goettel|first3=James T.|last4=Schrobilgen|first4=Gary J.|last5=Su|first5=Jing|last6=Li|first6=Jun|last7=Schlöder|first7=Tobias|last8=Riedel|first8=Sebastian|title=Identification of an iridium-containing compound with a formal oxidation state of IX|journal=Nature|date=22 October 2014|volume=514|issue=7523|pages=475–477|doi=10.1038/nature13795|pmid=25341786|bibcode=2014Natur.514..475W|s2cid=4463905}}</ref>

Argon methyl cations, (or methyliumargon) Ar<sub>x</sub>CH<sub>3</sub><sup>+</sup> are known for n=1 to 8. CH<sub>3</sub><sup>+</sup> is a Y shape, and when argon atoms are added they go above and below the plane of the Y. If more argon atoms are added they line up with the hydrogen atoms. Δ''H''<sup>0</sup> for ArCH<sub>3</sub><sup>+</sup> is 11 kcal/mol, and for Ar<sub>2</sub>CH<sub>3</sub><sup>+</sup> it is 13.5&nbsp;kcal/mol (for 2Ar + CH<sub>3</sub><sup>+</sup>).<ref>{{cite journal|last1=Hiraoka|first1=Kenzo|last2=Kudaka|first2=Ichiro|last3=Yamabe|first3=Shinichi|title=A charge transfer complex CH+3—Ar in the gas phase|journal=Chemical Physics Letters|date=March 1991|volume=178|issue=1|pages=103–108|doi=10.1016/0009-2614(91)85060-A|bibcode=1991CPL...178..103H}}</ref>

Boroxyl ring cationic complexes with argon [ArB<sub>3</sub>O<sub>4</sub>]<sup>+</sup>, [ArB<sub>3</sub>O<sub>5</sub>]<sup>+</sup>, [ArB<sub>4</sub>O<sub>6</sub>]<sup>+</sup> and [ArB<sub>5</sub>O<sub>7</sub>]<sup>+</sup> were prepared ''via'' a laser vaporization at cryogenic temperatures and investigated by infrared gas phase spectroscopy.<ref name=":0" /> They were the first large stable gas phase complexes that feature strong dative bonding between argon and boron.

=== Dications ===
Dications with argon are known for the coinage metals. Known dications include CuAr<sub>n</sub><sup>2+</sup> and AgAr<sub>n</sub><sup>2+</sup> for n=1-8, with a peak occurrence of CuAr<sub>4</sub><sup>2+</sup>, or AgAr<sub>4</sub><sup>2+</sup>, and AuAr<sub>n</sub><sup>2+</sup> n=3–7.  In addition to the four argon atoms, the six argon atoms clusters have enhanced concentration. The stability of the ions with two positive charges is unexpected as the ionization energy of argon is lower than the second ionization energy of the metal atom.  So the positive second charge on the metal atom should move to the argon, ionizing it, and then forming a highly repulsive molecule that undergoes a Coulomb explosion. However these molecules appear to be kinetically stable, and to transfer the charge to an argon atom, they have to pass through a higher energy state.<ref name="walk">{{cite journal|last1=Walker|first1=N. R.|last2=Wright|first2=R. R.|last3=Barran|first3=P. E.|last4=Cox|first4=H.|last5=Stace|first5=A. J.|title=Unexpected stability of [Cu⋅Ar][sup 2+], [Ag⋅Ar][sup 2+], [Au⋅Ar][sup 2+], and their larger clusters|journal=The Journal of Chemical Physics|date=2001|volume=114|issue=13|pages=5562|doi=10.1063/1.1352036|bibcode=2001JChPh.114.5562W|doi-access=free}}</ref> The clusters with four argon atoms are expected to be square planar, and those with six, to be octahedral distorted by the [[Jahn–Teller effect]].

{|class="wikitable"
!Ion
!Metal first ionization energy<br>eV
!Metal second ionization<br>eV
!binding energy<br>eV<ref name="walk"/>
!Dissociation energy<br>(kJ/mol)
!Bond length<br>(Å)
|-
|Cu<sup>2+</sup>Ar
|7.73
|20.29
|0.439
|
|2.4
|-
|Ag<sup>2+</sup>Ar
|7.58
|21.5
|0.199
|
|2.6
|-
|Au<sup>2+</sup>Ar
|9.22
|20.5
|0.670
|
|2.6
|}

=== Polyatomic anions ===

[[File:BCN-Ar cluster.png|thumb|Ball-and-stick model of the complex of superelectrophilic anion [B<sub>12</sub>(CN)<sub>11</sub>]<sup>−</sup> with Ar. B<sub>12</sub> core has nearly [[icosahedral]] symmetry. B – pink, C – grey, N – dark blue, Ar – blue.]]

Examples of anions containing strong bonds with noble gases are extremely rare: generally nucleophilic nature of anions results in their inability to bind to noble gases with their negative [[electron affinity]]. However, the 2017 discovery of "[[superelectrophilic anion]]s",<ref>{{Cite journal|last1=Rohdenburg|first1=Markus|last2=Mayer|first2=Martin|last3=Grellmann|first3=Max|last4=Jenne|first4=Carsten|last5=Borrmann|first5=Tobias|last6=Kleemiss|first6=Florian|last7=Azov|first7=Vladimir A.|last8=Asmis|first8=Knut R.|last9=Grabowsky|first9=Simon|date=2017|title=Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12Cl11]−|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=27|pages=7980–7985|doi=10.1002/anie.201702237|pmid=28560843|issn=1521-3773}}</ref> gas phase fragmentation products of ''closo''-[[dodecaborate]]s, led to the observation of stable anionic compounds containing a boron-noble gas bond with significant degree of covalent interaction. The most reactive superelectrophilic anion [B<sub>12</sub>(CN)<sub>11</sub>]<sup>−</sup>, fragmentation product of cyanated cluster [B<sub>12</sub>(CN)<sub>12</sub>]<sup>2-</sup>, was reported to bind argon spontaneously at room temperature.<ref name=":1" />

==Solid compounds==
Armand Gautier noticed that rock contained argon (and also nitrogen) that was liberated when the rock was dissolved in acid<ref>{{cite journal|last1=Gautier|first1=Armand|title=Chimie Geologique – Surl'existence d'azotures, argonures, arseniures, et iodures sans les roches crystalliniennes|journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences|date=January 1901|volume=132|page=934|url=http://gallica.bnf.fr/ark:/12148/bpt6k30888/f998.image|language=fr}}</ref> however how the argon was combined in rock was ignored by the scientific community.<ref name=posthumous/>

===Fullerene solvates===
Solid [[buckminsterfullerene]] has small spaces between the C<sub>60</sub> balls. Under 200&nbsp;MPa pressure and 200&nbsp;°C heat for 12 hours, argon can be intercalated into the solid to form crystalline Ar<sub>1</sub>C<sub>60</sub>. Once this cools down it is stable at standard conditions for months. Argon atoms occupy octahedral interstitial sites. The crystalline lattice size is almost unchanged at room temperature, but is slightly larger than pure C<sub>60</sub> below 265&nbsp;K. However argon does stop the buckyballs spinning below 250&nbsp;K, a lower temperature than in pure C<sub>60</sub>.<ref>{{cite journal|last1=Gadd|first1=G. E.|last2=Moricca|first2=S.|last3=Kennedy|first3=S. J.|last4=Elcombe|first4=M. M.|last5=Evans|first5=P. J.|last6=Blackford|first6=M.|last7=Cassidy|first7=D.|last8=Howard|first8=C. J.|last9=Prasad|first9=P.|last10=Hanna|first10=J. V.|last11=Burchwood|first11=A.|last12=Levy|first12=D.|title=Novel rare gas interstitial fullerenes of C<sup>60</sup> with Ar, Kr and Xe|journal=Journal of Physics and Chemistry of Solids|date=November 1997|volume=58|issue=11|pages=1823–1832|doi=10.1016/S0022-3697(97)00096-6|bibcode=1997JPCS...58.1823G}}</ref>

Solid [[C70 fullerene|C<sub>70</sub> fullerene]] will also absorb argon under pressure of 200&nbsp;MPa and at a temperature of 200&nbsp;°C. C<sub>70</sub>·Ar has argon in octahedral sites and has the rock salt structure, with cubic crystals in which the lattice parameter is 15.001&nbsp;Å. This compares to the pure C<sub>70</sub> lattice parameter of 14.964&nbsp;Å, so the argon forces the crystals to expand slightly. The C<sub>70</sub> ellipsoidal balls rotate freely in the solid, they are not locked into position by extra argon atoms filling the holes. Argon gradually escapes over a couple of days when the solid is stored at standard conditions, so that C<sub>70</sub>·Ar is less stable than C<sub>60</sub>·Ar. This is likely to be due to the shape and internal rotation allowing channels through which Ar atoms can move.<ref>{{cite journal|last1=Gadd|first1=G. E.|last2=Elcombe|first2=M. M.|last3=Dennis|first3=J.|last4=Moricca|first4=S.|last5=Webb|first5=N.|last6=Cassidy|first6=D.|last7=Evans|first7=P. J.|title=Novel rare gas interstitial fullerenes of C<sub>70</sub>|journal=Journal of Physics and Chemistry of Solids|date=June 1998|volume=59|issue=6–7|pages=937–944|doi=10.1016/S0022-3697(98)00017-1|bibcode=1998JPCS...59..937G}}</ref>

When [[fullerene]]s are dissolved and crystallized from [[toluene]], solids may form with toluene included as part of the crystal. However, if this crystallization is performed under a high pressure argon atmosphere, toluene is not included, being replaced by argon. The argon is then removed from the resultant crystal by heating to produce unsolvated solid fullerene.<ref>{{cite journal|last1=Ujihara|first1=Yuki|last2=Takahashi|first2=Yutaka|title=Binary Solid Solution of C<sub>60</sub>–C<sub>70</sub> Prepared by Liquid–Liquid Interfacial Precipitation Method|journal=Journal of the Japan Institute of Metals|date=2011|volume=75|issue=12|pages=671–677|doi=10.2320/jinstmet.75.671|doi-access=free}}</ref>

===Clathrate===
Argon forms a clathrate with [[hydroquinone]] (HOC<sub>6</sub>H<sub>4</sub>OH)<sub>3</sub>•Ar.<ref name="frenk">{{Cite book|last1=Frenking|first1=Gernot|last2=Cremer|first2=Dieter|title=The chemistry of the noble gas elements helium, neon, and argon — Experimental facts and theoretical predictions|volume=73|pages=82|doi=10.1007/3-540-52124-0_2|url=https://sites.smu.edu/dedman/catco/publications/pdf/StrucBond_73_1990.pdf|series=Structure and Bonding|year=1990|isbn=978-3-540-52124-2}}</ref> When crystallised from benzene under a pressure of 20 atmospheres of argon, a well defined structure containing argon results.<ref>{{cite journal|last1=Powell|first1=Herbert Marcus|last2=Guter|first2=M.|title=An Inert Gas Compound|journal=Nature|date=6 August 1949|volume=164|issue=4162|pages=240–241|doi=10.1038/164240b0|pmid=18135950|bibcode=1949Natur.164..240P|s2cid=4134617}}</ref> An argon-[[phenol]] clathrate 4C<sub>6</sub>H<sub>5</sub>OH•Ar is also known. It has a binding energy of 40 kJ/mol.<ref name=posthumous>{{cite book|last1=Edwards|first1=Kathleen F.|last2=Liebman|first2=Joel F.|chapter=Neil Bartlett: No Nobel for Noble Gases – Some Guesses Why|title=The Posthumous Nobel Prize in Chemistry. Volume 1. Correcting the Errors and Oversights of the Nobel Prize Committee|date=5 October 2017|pages=261–281|isbn=9780841232518|doi=10.1021/bk-2017-1262.ch012}}</ref> Other substituted phenols can also crystallise with argon.<ref name="frenk"/>
The argon water clathrate is described in the [[#Aqueous argon|Aqueous argon]] section.

===Argon difluoride===
Argon difluoride, ArF<sub>2</sub>, is predicted to be stable at pressures over 57&nbsp;GPa. It should be an electrical insulator.<ref>{{cite journal|last1=Kurzydłowski|first1=Dominik|last2=Zaleski-Ejgierd|first2=Patryk|title=High-pressure stabilization of argon fluorides|journal=Phys. Chem. Chem. Phys.|date=2016|volume=18|issue=4|pages=2309–2313|doi=10.1039/C5CP05725F|pmid=26742478|arxiv=1509.00188|bibcode=2016PCCP...18.2309K|s2cid=119249355}}</ref>

===Ne<sub>2</sub>Ar and Ar<sub>2</sub>Ne===
At around 4&nbsp;K there are two phases where neon and argon are mixed as a solid: Ne<sub>2</sub>Ar and Ar<sub>2</sub>Ne.<ref>{{cite journal|last1=Layer|first1=M.|last2=Heitz|first2=M.|last3=Meier|first3=J.|last4=Hunklinger|first4=S.|title=On solidified mixtures of neon and argon—Evidence for the formation of Ne<sub>2</sub>Ar- and Ar<sub>2</sub>Ne-phases|journal=Europhysics Letters (EPL)|date=January 2005|volume=69|issue=1|pages=95–101|doi=10.1209/epl/i2004-10298-x|bibcode=2005EL.....69...95L|s2cid=250894931 }}</ref> With Kr, solid argon forms a disorganized mixture.<ref>{{cite journal|last1=Layer|first1=M.|last2=Netsch|first2=A.|last3=Heitz|first3=M.|last4=Meier|first4=J.|last5=Hunklinger|first5=S.|title=Mixing behavior and structural formation of quench-condensed binary mixtures of solid noble gases|journal=Physical Review B|date=17 May 2006|volume=73|issue=18|pages=184116|doi=10.1103/PhysRevB.73.184116|bibcode=2006PhRvB..73r4116L}}</ref>

===ArH<sub>4</sub>===
Under high pressure stoichiometric solids are formed with hydrogen and oxygen: Ar(H<sub>2</sub>)<sub>2</sub> and Ar(O<sub>2</sub>)<sub>3</sub>.<ref name=hemley>{{cite journal|last1=Hemley|first1=R. J.|last2=Dera|first2=P.|title=Molecular Crystals|journal=Reviews in Mineralogy and Geochemistry|date=1 January 2000|volume=41|issue=1|page=369|doi=10.2138/rmg.2000.41.12|bibcode=2000RvMG...41..335H}}</ref>

Ar(H<sub>2</sub>)<sub>2</sub> crystallises in the hexagonal C14 MgZn<sub>2</sub> [[Laves phase]]. It is stable to at least 200&nbsp;GPa, but is predicted to change at 250&nbsp;GPa to an AlB<sub>2</sub> structure. At even higher pressures the hydrogen molecules should break up followed by metallization.<ref name=hemley/>

===ArO and ArO<sub>6</sub>===
Oxygen and argon under pressure at room temperature form several different alloys with different crystal structures. Argon atoms and oxygen molecules are similar in size, so that a greater range of miscibility occurs compared to other gas mixtures. Solid argon can dissolve up to 5% oxygen without changing structure. Below 50% oxygen a [[hexagonal close packed]] phase exists. This is stable from about 3GPa to 8.5&nbsp;GPa. Typical formula is ArO. With more oxygen between 5.5 and 7&nbsp;GPa, a cubic ''Pm''3''n'' structure exists, but under higher pressure it changes to a ''I''{{overbar|4}}2''d'' space group form. With more than 8.5&nbsp;GPa these alloys separate to solid argon and ε-oxygen. The cubic structure has a unit cell edge of 5.7828&nbsp;Å at 6.9&nbsp;GPa. The representative formula is Ar(O<sub>2</sub>)<sub>3</sub>.<ref>{{cite journal|last1=Weck|first1=Gunnar|last2=Dewaele|first2=Agnès|last3=Loubeyre|first3=Paul|title=Oxygen/noble gas binary phase diagrams at 296 K and high pressures|journal=Physical Review B|date=28 July 2010|volume=82|issue=1|pages=014112|doi=10.1103/PhysRevB.82.014112|bibcode=2010PhRvB..82a4112W}}</ref>

===ArHe<sub>2</sub>===
Using [[density-functional theory]] ArHe<sub>2</sub> is predicted to exist with the MgCu<sub>2</sub> Laves phase structure at high pressures below 13.8&nbsp;GPa. Above 13.8&nbsp;GPa it transforms to AlB<sub>2</sub> structure.<ref>{{cite journal|last1=Cazorla|first1=C.|last2=Errandonea|first2=D.|last3=Sola|first3=E.|s2cid=49570874|title=High-pressure phases, vibrational properties, and electronic structure of Ne(He)<sub>2</sub> and Ar(He)<sub>2</sub>: A first-principles study|journal=Physical Review B|date=10 August 2009|volume=80|issue=6|pages=064105–1|doi=10.1103/PhysRevB.80.064105|bibcode=2009PhRvB..80f4105C}}</ref>

===Ar-TON===
Under pressure argon inserts into [[zeolite]]. Argon has an atomic radius of 1.8&nbsp;Å, so it can insert into pores if they are big enough. Each unit cell of the TON zeolite can contain up to 5 atoms of argon, compared to 12 of neon. Argon infused TON zeolite (Ar-TON) is more compressible than Ne-TON as the unoccupied pores become elliptical under increased pressure. When Ar-TON is brought to atmospheric pressure, the argon only desorbs slowly, so that some remains in the solid without external pressure for a day.<ref name=thib>{{cite journal |last1=Thibaud |first1=Jean-Marc |last2=Rouquette |first2=Jérôme |last3=Dziubek |first3=Kamil |last4=Gorelli |first4=Federico A. |last5=Santoro |first5=Mario |last6=Garbarino |first6=Gaston |last7=Clément |first7=Sébastien |last8=Cambon |first8=Olivier |last9=van der Lee |first9=Arie |last10=Di Renzo |first10=Francesco |last11=Coasne |first11=Benoît |last12=Haines |first12=Julien |title=Saturation of the Siliceous Zeolite TON with Neon at High Pressure |journal=The Journal of Physical Chemistry C |date=3 April 2018 |volume=122 |issue=15 |pages=8455–8460 |doi=10.1021/acs.jpcc.8b01827}}</ref>

===Nickel argide===
At 140&nbsp;GPa and 1500K nickel and argon form an alloy, NiAr.<ref>{{cite web |last1=Dalton |first1=Louisa |title=Argon reacts with nickel under pressure-cooker conditions |url=https://cen.acs.org/materials/inorganic-chemistry/Argon-reacts-nickel-under-pressure/97/web/2019/10 |website=Chemical & Engineering News |access-date=6 November 2019 |language=en |date=30 October 2019}}</ref> NiAr is stable at room temperature and a pressure as low as 99&nbsp;GPa. It has a [[face-centred cubic]] (fcc) structure. The compound is metallic. Each nickel atom loses 0.2 electrons to an argon atom which is thereby an oxidant. This contrasts with Ni<sub>3</sub>Xe, in which nickel is the oxidant. The volume of the ArNi compound is 5% less than that of the separate elements at these pressures. If this compound exists in the [[core of the Earth]] it could explain why only half the [[argon-40]] that should be produced during the radioactive decay that produces [[geothermal heating]] seems to exist on the Earth.<ref>{{cite journal |last1=Adeleke |first1=Adebayo A. |last2=Kunz |first2=Martin |last3=Greenberg |first3=Eran |last4=Prakapenka |first4=Vitali B. |last5=Yao |first5=Yansun |last6=Stavrou |first6=Elissaios |title=A High-Pressure Compound of Argon and Nickel: Noble Gas in the Earth's Core? |journal=ACS Earth and Space Chemistry |volume=3 |issue=11 |pages=2517–2524 |date=15 October 2019 |doi=10.1021/acsearthspacechem.9b00212|bibcode=2019ESC.....3.2517A |osti=1575040 |s2cid=210511220 }}</ref>

==Organoargon chemistry==
Organoargon chemistry describes the synthesis and properties of [[chemical compound]]s containing a [[carbon]] to [[argon]] [[chemical bond]].

Very few such compounds are known. The reaction of acetylene dications with argon produced {{chem2|HCCAr(2+)}} in 2008.<ref>{{cite journal |last1=Ascenzi |first1=Daniela |last2=Tosi |first2=Paolo |first3=Jana |last3=Roithová |first4=Claire L. |last4=Ricketts |first5=Detlef |last5=Schröder |first6=Jessica F. |last6=Lockyer |first7=Michael A. |last7=Parkes |first8=Stephen D. |last8=Price |date=2008 |title=Generation of the organo-rare gas dications HCCRg<sup>2+</sup> (Rg = Ar and Kr) in the reaction of acetylene dications with rare gases |url= |journal=Physical Chemistry Chemical Physics |volume=10 |issue= 47|pages=7121–7128 |doi=10.1039/B810398D |pmid=19039346 |bibcode=2008PCCP...10.7121A |access-date=}}</ref> Reaction of the {{chem2|CF3(2+)}} dication with argon produced {{chem2|ArCF2(2+)}}: this reaction is unique to argon among the noble gases.<ref>{{cite journal |last1=Lockyear |first1=Jessica F. |last2=Douglas |first2=Kevin |first3=Stephen D. |last3=Price |first4=Małgorzata |last4=Karwowska |first5=Karol J. |last5=Fijalkowski |first6=Wojciech |last6=Grochala |first7=Marek |last7=Remeš |first8=Jana |last8=Roithová |first9=Detlef |last9=Schröder |date=2010 |title=Generation of the ArCF<sub>2</sub><sup>2+</sup> Dication |url= |journal=The Journal of Physical Chemistry Letters |volume=1 |issue=1 |pages=358–362 |doi=10.1021/jz900274p |access-date=}}</ref>

The compound FArCCH has been theoretically studied and is predicted to be stable.<ref>{{cite journal |last1=Cohen |first1=Arik |last2=Lundell |first2=Jan |first3=R. Benny |last3=Gerber |date=2003 |title=First compounds with argon–carbon and argon–silicon chemical bonds |url= |journal=Journal of Chemical Physics |volume=119 |issue=13 |pages=6415–6417 |doi=10.1063/1.1613631 |bibcode=2003JChPh.119.6415C |access-date=}}</ref> FArCCF might also be stable enough to synthesise and detect, but probably not FArCCArF.<ref>{{cite journal |last1=Yockel |first1=Scott |last2=Gawlik |first2=Evan |first3=Angela K. |last3=Wilson |date=2007 |title=Structure and Stability of the Organo-Noble Gas Molecules XNgCCX and XNgCCNgX (Ng = Kr, Ar; X = F, Cl)† |url= |journal=The Journal of Physical Chemistry A |volume=111 |issue=44 |pages=11261–11268 |doi=10.1021/jp071242p |pmid=17880047 |bibcode=2007JPCA..11111261Y |access-date=}}</ref> Calculations in 2015 suggest that FArCCH and FArCH<sub>3</sub> are stable, but not FArCN.<ref>{{cite journal |last1=Mohajeri |first1=Afshan |last2=Bitaab |first2=Nafiseh |date=2014 |title=Investigating the nature of intermolecular and intramolecular bonds in noble gas containing molecules |url= |journal=International Journal of Quantum Chemistry |volume=115 |issue=3 |pages=165–171 |doi=10.1002/qua.24804 |access-date=}}</ref> {{chem2|FArCC(-)}} should be kinetically stable, as is also expected of the krypton and xenon (but not helium) analogues.<ref>{{cite journal |last1=Peng |first1=Chia-Yu |last2=Yang |first2=Chang-Yu |first3=Yi-Lun |last3=Sun |first4=Wei-Ping |last4=Hu |date=2012 |title=Theoretical prediction on the structures and stability of the noble-gas containing anions FNgCC<sup>−</sup> (Ng=He, Ar, Kr, and Xe) |url= |journal=The Journal of Chemical Physics |volume=137 |issue=19 |page=194303 |doi=10.1063/1.4766326 |pmid=23181302 |bibcode=2012JChPh.137s4303P |access-date=}}</ref> HArC<sub>4</sub>H (for which the krypton analogue is known) and HArC<sub>6</sub>H have also been predicted as stable.<ref>{{cite journal |last1=Sheng |first1=Li |last2=Cohen |first2=Arik |first3=R. Benny |last3=Gerber |date=2006 |title=Theoretical Prediction of Chemically Bound Compounds Made of Argon and Hydrocarbons |url= |journal=Journal of the American Chemical Society |volume=128 |issue=22 |pages=7156–7157 |doi=10.1021/ja0613355 |pmid=16734457 |access-date=}}</ref> FArCO<sup>+</sup> and ClArCO<sup>+</sup> should be metastable and might be possible to characterise under cryogenic conditions.<ref>{{cite journal |last1=Manna |first1=Debashree |last2=Ghosh |first2=Ayan |first3=Tapan K. |last3=Ghanty |date=2013 |title=Theoretical Prediction of XRgCO<sup>+</sup> Ions (X = F, Cl, and Rg = Ar, Kr, Xe) |url= |journal=The Journal of Physical Chemistry A |volume=117 |issue=51 |pages=14282–14292 |doi=10.1021/jp410631y |pmid=24295279 |bibcode=2013JPCA..11714282M |access-date=}}</ref> Calculations suggest that HArCCF and HCCArF should be stable, and that HNgCCF molecules should be more stable than HNgCCH (Ng = Ar, Kr, Xe); the corresponding krypton species have been experimentally produced, but not the argon species despite an experimental attempt. HCCNgCN and HCCNgNC (Ng = Ar, Kr, Xe) are likewise computed to be stable, but experimental searches for them have failed.<ref>{{cite journal |last1=Khriachtchev |first1=Leonid |last2=Domanskaya |first2=Alexandra |first3=Jan |last3=Lundell |first4=Alexander |last4=Akimov |first5=Markku |last5=Räsänen |first6=Eugenii |last6=Misochko |date=2010 |title=Matrix-Isolation and ab Initio Study of HNgCCF and HCCNgF Molecules (Ng = Ar, Kr, and Xe) |url= |journal=The Journal of Physical Chemistry A |volume=114 |issue=12 |pages=4181–4187 |doi=10.1021/jp1001622 |pmid=20205379 |bibcode=2010JPCA..114.4181K |access-date=|hdl=10138/23938 |hdl-access=free }}</ref>

==References==
{{Reflist|30em}}

==External links==
*{{cite journal|url=https://www.science.org/doi/10.1126/science.1074201|title=Rotationally Resolved Infrared Spectrum of the Charge Transfer Complex [Ar–N<sub>2</sub>]<sup>+</sup> Supporting Online Material Observed frequencies and observed-calculated values of rotational transitions in the <sup>2</sup>Σ<sup>+</sup> ground state of [ArN<sub>2</sub>]<sup>+</sup> upon excitation of the NN stretching fundamental|journal=Science|date=16 August 2002|volume=297|issue=5584|pages=1166–1167|doi=10.1126/science.1074201|last1=Linnartz|first1=H.|last2=Verdes|first2=D.|last3=Maier|first3=J. P.|pmid=12183626|bibcode=2002Sci...297.1166L |s2cid=45850131}}
*{{cite journal|doi=10.1016/0584-8547(94)80084-7|title=Some characteristics of polyatomic ion spectra in inductively coupled plasma mass spectrometry|journal=Spectrochimica Acta Part B: Atomic Spectroscopy|volume=49|issue=10|pages=955|year=1994|last1=Nonose|first1=Naoko S.|last2=Matsuda|first2=Naoki|last3=Fudagawa|first3=Noriko|last4=Kubota|first4=Masaaki|bibcode=1994AcSpe..49..955N}}
*{{cite journal|doi=10.1039/C4RA13102A|pmid=26702354|title=Ion kinetics in Ar/H<sub>2</sub> cold plasmas: The relevance of ArH<sup>+</sup>|journal=RSC Adv.|volume=4|issue=107|pages=62030–62041|year=2014|last1=Jiménez Redondo|first1=Miguel|last2=Cueto|first2=Maite|last3=Doménech|first3=José Luis|last4=Tanarro|first4=Isabel|last5=Herrero|first5=Víctor J.|pmc=4685740|bibcode=2014RSCAd...462030J}}

{{Noble gas compounds}}
{{Chemical compounds by element}}

[[Category:Argon compounds| ]]
[[Category:Chemical compounds by element]]
[[Category:Argon|Compounds]]